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First published 1993 

Reprinted with corrections 1994, 1995 

Second edition 1997 

Reprinted 1999, 2000, 2001,2002, 2004, 2006 

Copyright © 1993, 1997, C. M. Lalli andT. R. Parsons. All rights reserved 

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British Library Cataloguing in Publication Data 

Lalli, Carol M. 

Biological oceanography: an introduction. - 2nd ed. 

1. Marine biology 2. Marine ecology 

I. Title II. Parsons, Timothy R. (Timothy Richard), 1932— 


Library of Congress Cataloguing in Publication Data 
Lalli, Carol M. 

Biologocial oceanography: an introduction / Carol M. Lalli and 
Timothy R. Parsons. - 2nd ed. 
p. cm 

Ineludes bibliographical references and Índex. 

1. Marine biology 2. Marine ecology 3. Oceanography. 

I. Parsons, Timothy Richard, 1932- II. Title 

QH91.L35 96-42139 

574.92-dc20 CIP 

ISBN 0 7506 3384 0 

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1.3.1 r- and /C-selection 5 





2.1.1 Radiation at the sea surface 17 

2.1.2 Radiation in the sea 18 


2.2.1 Sea surface temperatures 21 

2.2.2 Vertical temperature distribution 24 

2.3 SALINITY 25 

2.3.1 Range and distribution of salinity 26 

2.3.2 Biological importance of salinity 28 

2.4 DENSITY 30 

2.5 PRESSURE 33 


2.6.1 Biological significance of currents 35 




3.1.1 Diatoms 40 

3.1.2 Dinoflagellates 42 

3.1.3 Other phytoplankton 45 


3.2.1 Methods of measuring biomass and primary productivity 48 




3.5.1 Oceanic gyres and rings 60 


Continental convergence and divergence 



Planetary frontal systems 



Shelf-break fronts 



River-plume fronts 



Island mass effect and Langmulr frontal zones 

















































The microbial loop 






Fleld studies 



Experimental blologlcal oceanography 

































6.5 SEABIRDS 153 

6.6 MARINE FISH 156 

6.6.1 Fish migrations 162 


6.7.1 World fish catch and fisheries management 163 

6.7.2 Fluctuations in the abundance of fish stocks 164 

6.7.3 Regulation of recruitment and growth in fish 167 

6.7.4 Fishing and the use of near real-time oceanographic data 170 





7.1.1 Measurements of benthic primary production 180 


7.2.1 Systematics and biology 182 

7.2.2 Sampling and production measurements 191 





8.1.1 Tides 197 

8.1.2 Environmental conditions and adaptations of intertidal organisms 197 


8.2.1 Zonation 198 

8.2.2 Trophic relations and the role of grazing and predation in determining 

community structure 201 



8.4.1 Environmental characteristics 205 

8.4.2 Species composition 206 

8.5 ESTUARIES 209 

8.6 CORAL REEFS 213 

8.6.1 Distribution and limiting factors 213 

8.6.2 Coral structure 214 

8.6.3 Diversity 215 

8.6.4 Nutrition and production in reefs 217 

8.6.5 Production estimates 218 

8.6.6 Formation and growth of reefs 219 

8.6.7 Zonation patterns on reefs 221 


8.7.1 What are mangroves? 223 

8.7.2 Ecological features of mangrove swamps 224 

8.7.3 Importance and uses of mangroves 225 


8.8.1 Faunal composition 227 

8.8.2 Species diversity 231 

8.8.3 Biomass 232 

8.8.4 Food sources 233 

8.8.5 Rafes of biological processes 236 

8.8.6 Future prospects 238 


8.9.1 Chemosynthetic production 239 

8.9.2 Vent fauna 239 

8.9.3 Shallow vents and coid seeps 241 

8.9.4 Unique environmental features of sulphide communities 242 





9.2.1 Petroleum hydrocarbons 252 

9.2.2 Plastics 252 

9.2.3 Pesticides and other biologically active organic compounds 253 

9.2.4 Heavy metáis 254 

9.2.5 Sewage 255 

9.2.6 Radioactive wastes 255 

9.2.7 Thermal effluents 256 



9.4.1 Estuaries 258 

9.4.2 Mangrove swamps 259 

9.4.3 Coral reefs 260 









INDEX 307 

This Page Intentionally Left Blank 


This volume is complementary to the Open University Series on 
oceanography. It is designed so that it can be read on its own, like any other 
textbook, or studied as part of S330 Oceanography , a third level course for 
Open University students. The Science of oceanography as a whole is 
multidisciplinary. However, different aspects fall naturally within the scope 
of one or other of the major ‘traditionaF disciplines.Thus, you will get the 
most of this volume if you have some previous experience of studying 
biology. The other volumes in the Open University Series lie more within the 
fields of physics, geology or chemistry (and their associated sub-branches). 

Chapter 1 begins by describing unique properties that affect life in the sea, 
and by making comparisons with life on land. The major categories used to 
define marine environments and marine organisms are introduced, and basic 
ecological terms and concepts that are central to studies of biológica! 
oceanography are reviewed. The last section of Chapter 1 outlines the 
historical development of this scientific discipline. 

Chapter 2 considers some physical and Chemical features of the oceans 
including light, temperature, salinity, density, and pressure, all of which 
greatly influence the conditions under which marine organisms live. Major 
water current patterns are described because they transport many marine 
organisms, as well as dissolved gases and other Chemical substances, and 
thus they affect the distributions of species and the size of populations in 
particular areas. 

Chapter 3 introduces the various types of phytoplankton, and describes the 
way these floating plants manufacture energy-rich organic compounds by the 
process of photosynthesis. Plants require energy from solar radiation, and the 
effects of diminishing light levels with increasing depth are considered in 
detail. Essential nutrients like nitrate and phosphate are present in relatively 
low amounts in the lighted surface waters of the ocean, and the 
consequences of varying nutrient concentrations on plant growth are 
explored. Simple mathematical expressions are used to describe the 
relationships between plant growth and light intensity or nutrient 
concentration. Finally, vertical water movements that cause geographic 
differences in biological productivity are examined. 

Chapter 4 describes the major types of zooplankton and their general life 
history patterns and feeding mechanisms. Vertical distribution of these 
animáis is considered in relation to environmental differences with depth. 
Many animáis migrate vertically in the water column., either daily or 
seasonally, and the consequences of these migrations are considered. Broad 
geographic patterns of distribution are also described, as well as smaller 
scale patterns established by a variety of physical and biological influences. 
Lastly, there is a discussion of long-term (decadal) changes in the abundance 
and species composition of zooplankton communities. 

Chapter 5 explores the flow of energy through marine food chains and food 
webs, and explains the importance of using these ecological concepts to 
predict yields of fish based on measurements of plant growth in particular 
geographic areas. Different experimental approaches are also described that 
can be used to study biological oceanographic issues concemed with the 
dynamics of marine food chains. Finally, the important Chemical changes 

that occur during the recycling of minerals are considerad, with particular 
emphasis on nitrogen and carbón cycles. 

Chapter 6 begins by describing the various types of nekton including the 
larger crustaceans, squid, marine reptiles, marine mammals, seabirds, and 
fish. The ecological importance of these larger animáis is emphasized, as 
well as the consequences of their exploitation. The second half of this 
chapter looks at fisheries management problems and at the causes of 
fluctuation in the abundance of fish stocks. 

Chapter 7 describes the major types of plants and animáis that live on the 
seafloor, concluding with a section that explains how these organisms are 
sampled, and how their rate of growth (or production) can be determined. 

Chapter 8 reviews the environmental conditions and general ecology of 
different bottom communities ranging from températe intertidal communities, 
to tropical coral reefs and mangrove swamps, to deep-sea assemblages. 

Chapter 9 considers the many ways in which human populations cause 
changes in marine ecosystems. Attention is given to problems of overfishing 
and exploitation of marine resources, and to the impacts of various types of 
pollution. There is also consideration of the changes caused by the accidental 
or delibérate introduction of marine species into new environments. Specific 
impacts on estuaries, mangrove swamps, and coral reefs are discussed. 

You will find questions designed to help you to develop arguments and/or 
test your own understanding as your read, with answers provided at the back 
of this volume. Important technical terms are printed in bold type where 
they are first introduced or defined. 



The oceans occupy about 71% of the Earth’s surface. The deepest parts of 
the seafloor are almost 11 000 m from the sea surface, and the average depth 
of the oceans is about 3800 m. The total volume of the marine environment 
(about 1370 x 10 6 km 3 ) provides approximately 300 times more space for 
life than that provided by land and freshwater combined. The ñame given to 
our planet, ‘Earth\ is a synonym for dry land, but it is a misnomer in that it 
does not describe the dominant feature of the planet — which is a vast 
expanse of blue water. 

The age of Earth is thought to be about 4600 million years. The ocean and 
atmosphere formed as the planet cooled, some time between 4400 and 3500 
million years ago, the latter date marking the appearance of the first forms of 
life (see the Geologic Time Scale, Appendix 1). The earliest organisms are 
believed to have originated in the ancient oceans, many millions of years 
before any forms of life appeared on dry land. All known phyla (both extinct 
and extant) originated in the sea, although some later migrated into 
freshwater or terrestrial environments. Today there are more phyla of animáis 
in the oceans than in freshwater or on land, but the majority of all described 
animal species are non-marine. The difference in the number of species is 
believed to be due largely to the greater variety of habitats on land. 


Why should life have arisen in the sea* and nol on land? 

Marine and terrestrial environments provide very different physical 
conditions for life. Seawater has a much higher density than air, and 
consequently there is a major difference in the way gravity affects organisms 
living in seawater and those living in air. Whereas terrestrial plants and 
animáis generally require large proportions of skeletal material (e.g. tree 
trunks, bones) to hold themselves erect or to move against the forcé of 
gravity, marine species are buoyed up by water and do not store large 
amounts of energy in skeletal material. The majority of marine plants are 
microscopic, floating species; many marine animáis are invertebrates without 
massive skeletons; and fish have small bones. Floating and swimming 
require little energy expenditure compared with walking or flying through 
air. Overcoming the effects of gravity has been energetically expensive for 
terrestrial animáis, and perhaps it should not be surprising that the first forms 
of life and all phyla evolved where the buoyancy of the environment 
permitted greater energy conservation. 

Two other features of the ocean are especially conducive to life. Water is a 
fundamental constituent of all living organisms, and it is cióse to being a 
universal solvent with the ability to dissolve more substances than any other 
liquid. Whereas water can be in short supply on land and thus limiting to 
life, this is obviously not the case in the marine environment. Secondly, the 
temperature of the oceans does not vary as drastically as it does in air. 


On the other hand, certain properties of the sea are less favourable for life 
than conditions on land. Plant growth in the sea is limited by light because 
only about 50% of the total solar radiation actually penetrates the sea 
surface, and much of this disappears rapidly with depth. Marine plants can 
grow only within the sunlit surface región, which extends down to a few 
metres in turbid water or, at the most, to several hundred metres depth in 
clear water. The vast majority of the marine environment is in perpetual 
darkness, yet most animal life in the sea depends either directly or indirectly 
on plant production near the sea surface. Marine plant growth is also limited 
by the availability of essential nutrients, such as nitrates and phosphates, that 
are present in very small quantities in seawater compared with 
concentrations in soil. On land, nutrients required by plants are generated 
nearby from the decaying remains of earlier generations of plants. In the sea, 
much decaying matter sinks to depths below the surface zone of plant 
production, and nutrients released from this material can only be returned to 
the sunlit area by physical movements of water. 

The greatest environmental fluctuations occur at or near the sea surface, 
where interactions with the atmosphere result in an exchange of gases, 
produce variations in temperature and salinity, and create water turbulence 
from winds. Deeper in the water column, conditions become more constant. 
Vertical gradients in environmental parameters are predominant features of 
the oceans, and these establish depth zones with different types of living 
conditions. Not only does light diminish with depth, but temperature 
decreases to a constant valué of 2-4°C, and food becomes increasingly 
scarce. On the other hand, hydrostatic pressure increases with depth, and 
nutrients become more concentrated. Because of the depth-related changes in 
environmental conditions, many marine animáis tend to be restricted to 
distinctive vertical zones. On a horizontal scale, geographic barriers within 
the water column are set by physical and Chemical differences in seawater. 

Much of this text deais with descriptions of marine communities and the 
interactions between physical, Chemical and biological properties that 
determine the nature of these associations. Some attention is also given to 
the exploitation of marine, biological resources. Despite the fact that the 
oceans occupy almost three-quarters of the Earth’s surface, only 2% of the 
present total human food consumption comes from marine species. However, 
this is an important nutritional source because it represents about 20% of the 
high-quality animal protein consumed in the human diet. Although a greater 
total amount of organic matter is produced annually in the ocean than on 
land, the economic utilization of the marine production is much less 
effective. One branch of biological oceanography, fisheries oceanography, is 
a rapidly developing field that addresses the issue of fish production in 
the sea. 


The world’s oceans can be subdivided into a number of marine 
environments (Figure 1.1). The most basic división separates the pelagic and 
benthic realms. The pelagic environment (pelagic meaning ‘open sea’) is 
that of the water column, from the surface to the greatest depths. The 
benthic environment (benthic meaning ‘bottom’) encompasses the seafloor 


supralittoral high 

water | 0 w water 

(plankton + nekton) 





koQ m 


^1000 m 



.v 4000 m 


6000 m 

10 000 m 


Figure 1.1 The basic ecological divisions of 
the ocean. The neritic (or inshore) petagic zone 
is separated from the oceanic (or offshore) 
pelagic zone by the edge of the continental 
sheíf, whích ís generally at about 200 m depth. 
Benthíc habitats are in bold type; pelagic 
divisions are in blue. (Not to scale.) 

and ineludes such areas as shores, littoral or intertidal areas, coral reefs, and 
the deep seabed. 

Another basic división separates the vast open ocean, the oceanic 
environment, from the inshore neritic zone. This división is based on depth 
and distance from land, and the separation is conventionally made at the 
200 m depth limit which generally marks the edge of the continental shelf 
(Figure 1.1). In some areas like the west coast of South America where the 
shelf is very narrow, the neritic zone will extend only a very slight distance 
from shore. In other areas (e.g. off the north-east coast of the United States), 
the neritic zone may extend several hundred kilometres from land. Overall, 
continental shelves underlie about 8% of the total ocean, an area equal to 
about that of Europe and South America combined. 

Further divisions of the pelagic and benthic environments can be made 
which divide them into distinctive ecological zones based on depth and/or 
bottom topography. These will be considered in later chapters. 

Marine organisms can be classified according to which of the marine 
environments they inhabit. Thus there are oceanic species and neritic 
species depending upon whether the organisms are found in offshore or 
Coastal waters, respectively. Similarly, plants or animáis that live in 
association with the seafloor are collectively called benthos. The benthos 
ineludes attached seaweeds, sessile animáis like sponges and barnacles, and 
those animáis that crawl on or burrow into the substrate. Additional 
subdivisions of the benthos are given in Chapter 7. 

The pelagic environment supports two basic types of marine organisms. One 
type comprises the plankton, or those organisms whose powers of 

locomotion are such that they are incapable of making their way against a 
current and thus are passively transported by currents in the sea. The word 
plankton comes from the Greek planktos , meaning that which is passively 
drifting or wandering. Depending upon whether a planktonic organism is a 
plant or animal, a distinction is made between phytoplankton and 
zooplankton. Although many planktonic species are of microscopic 
dimensions, the term is not synonymous with small size as some of the 
zooplankton inelude jellyfish of several metres in diameter. Ñor are all 
plankton completely passive; most, including many of the phytoplankton, are 
capable of swimming. The remaining inhabitants of the pelagic environment 
form the nekton. These are free-swimming animáis that, in contrast to 
plankton, are strong enough to swim against currents and are therefore 
independent of water movements. The category of nekton ineludes fish, 
squid, and marine mammals. 




0 02-0 2 nm 



0 2-2 Onm 

2.0-20 n/m 


20-200 nin 


0.2-20 mm 

2-20 cm 

20-200 cm 




2-20 cm 



2-20 dm 



2-20 m 
















Figure 1.2 A grade scale for the size classification of pelagic organisms. 

Finally, it is often convenient to characterize pelagic organisms according to 
size. Figure 1.2 presents one scheme for dividing plankton and nekton into 
size categories. The categories encompass the smallest and most primitive 
members of the pelagic environment, the viruses and bacteria, which are 
placed in the femtoplankton (<0.2 /zm) and picoplankton (0.2-2.0 /zm) 
categories, respectively. The largest nekton form the other extreme of the 
size range, with species of whales that can be 20 m or more in length. 
Phytoplankton and zooplankton (the latter including both unicellular 
protozoans and multicellular metazoans) comprise the intermediate size 

QUE5TI0N 1.1 In Figure L2, why is there overlap in size between the largest 
plankton (macro- and megapJanklon) and the smallest nekton? 


Basic ecological concepts are central to many studies of biological 
oceanography, and certain ecological terms will be used throughout this text. 
Marine organisms can be considered either individually or, more commonly 
in ecological studies, on different collective levels. A species is defined as a 

popularon [JenSftv 



Figure 1.3 A iogistic growth curve. Note that 
the population initially grows rapidly, then it 
slows and eventually ceases to grow as it 
reaches the carrying capacity of the 
environment. K is the maximal population size 
at carrying capacity, and r is the rate of 
population growth. 

distinctive group of interbreeding individuáis that is reproductively isolated 
from other such groups. A population refers to a group of individuáis of 
one species living in a particular place, and population density refers to the 
number of individuáis per unit area (or per unit volume of water). The 
various populations of micro-organisms, plants, and animáis that inhabit the 
same physical area make up an ecological community. 

The habitat of an organism is the place where it lives, but the term also may 
refer to the place occupied by an entire community. The environment 
consists of both nonliving abiotic (physical and Chemical) components like 
temperature and nutrient concentrations, and biotic components that inelude 
the other organisms and species with which an organism interaets (e.g. 
predators, parasites, competitors, and mates). 

The highest level of ecological integration is the ecosystem, which 
encompasses one or more communities in a large geographic area and 
ineludes the abiotic environment in which the organisms live. Examples of 
ecosystems could inelude estuaries (see Section 8.5 for the component 
communities of estuaries), or the total pelagic water column (with different 
communities at different depths). Species diversity is often used to describe 
the simplicity (or complexity) of communities and ecosystems; it can be 
defined in several ways but, unless otherwise stated, the term is used 
throughout this book to mean total number of species. 

SI units (International System of Units) are now widely used in Sciences 
since the system was adopted by the Conférence générale des poids et 
mesures (CGPM) in 1960. However, much of the ecological literature 
relevant to biológica! oceanography has used other types of units, and we 
have continued to report results as they were originally published. 
Conversions for more commonly used units can be found in Appendix 2. 

1.3.1 r- AND ff-SELECTION 

Each plant or animal species, whether pelagic or benthic, or marine or 
terrestrial, has its own unique suite of biological and ecological features that 
define its life history. Two very different types of life histories are 
recognized as representing the extremes along a broad continuum of patterns. 
One type is known as r-selected, and this life history pattern is 
demonstrated by opportunistic species. The contrasting type of life history 
is called K -selected, and organisms with this complement of characteristics 
are referred to as equilibrium species. The terms r and K refer to different 
portions of growth curves (Figure 1.3); r is the intrinsic rate at which a 
population can increase, and K is the máximum population density that can 
be supported by the environment. Populations that are kept at low densities 
by abiotic or biotic environmental factors are influenced largely by the 
parameter r, whereas populations that are at or near the carrying capacity of 
the environment will be influenced by the parameter K. 

Generally, r-selected species are of relatively small size, and they reach 
sexual maturity early. They usually produce many young several times per 
year. These opportunistic species typically live in variable or unpredictable 
environments, and they are able to respond quickly to favourable conditions 
or new habitats by rapid rates of colonization and reproduction. However, 
r-selected species typically have little ability to compete with other species, 
and they have high mortality. Consequently, populations of opportunistic 
species tend to be short-lived. r-selection leads to high biological 


productivity, as the organisms devote a large proportion of their available 
energy to rapid growth and reproduction. 

At the other extreme, fairly constant and/or predictable habitats favour 
K-selected species that are larger in size and slow-growing, but long-lived. 
They take longer to reach reproductive maturity and produce fewer young, 
but death rates are fairly low. They are particularly adapted to live in areas 
that are not subject to frequent disturbance, as they require sufficient time to 
complete their life cycles. Equilibrium species tend to build up their 
population sizes to the máximum that the environment will sustain (i.e. to 
the carrying capacity of the environment), and they sustain this population 
size for long periods by utilizing their resources very efficiently. 

Table 1.1 summarizes other differences in these very different life histories. 
As mentioned above, the concept of r- and Á'-selection can be applied to 
pelagic as well as to benthic marine organisms, and it is also used in 
terrestrial ecology. It should be noted that the different pattems are applied 
in a relative sense to organisms. For example, although phytoplankton as a 
whole have short life cycles compared to whales, within the group there are 
both r-selected and /T-selected phytoplankton species. It should further be 
stressed that the majority of organisms possess a mixture of r- and 
tf-features. However, the importance of examining contrasting life styles lies 
in recognition of the ways in which different species deal with competition, 
predation, and environmental change. Throughout this book, you will be 
encouraged to learn about the many different life history pattems of marine 
plants and animáis, and to compare them on the basis of r- and K- strategies. 

Table 1.1 A comparison of the life history pattems exhibited by r- and /T-selected 
marine species. 


opportunistic species 

equilibrium species 

C1 i mate 



Adult size 



Growth rate 

Time of sexual 










Number of young 



Dispersal ability 



Population size 

variable; usually 

relatively constant; 

below carrying 

at or near carrying 

capacity of 







high; independent 

lower; density 


of population 


Life span 

short (<1 yr) 

long (>1 yr) 

Pelagic/benthic ratio 





Human interest in the biology of the oceans can be traced back to 
observations made in the fourth century B.C. by Aristotle, who described and 
catalogued 180 species of marine animáis. The great sea-going expeditions 
of the fifteenth and sixteenth centuries increased geographical knowledge of 
the oceans and added incidental observations on biology, but modera studies 
on the biology of the oceans did not really start until the middle of the 
nineteenth century. 

The British naturalist, Edward Forbes (1815-54) (Figure 1.4), is often 
credited with being a founding father of oceanography, as he was one of the 
first persons to conduct systematically designed studies of the marine biota. 
He pioneered in the use of a dredge for obtaining samples of benthic marine 
animáis, and he recognized that different species occupy different depth 
zones. His book, The Natural History of the European Seas , was published 
five years after his death, at the same time as Darwin’s The Origin of 
Species. Unfortunately, Forbes is often remembered for the 1843 publication 
of his azoic hypothesis, which claimed that marine organisms could not 
exist at depths exceeding about 300 fathoms (550 m). Forbes was unaware 
that there were already records of life from deeper areas of the sea. In 1818, 
John Ross had obtained bottom samples containing worms and a starfish 
from about 1920 m in Baffin Bay, west of Greenland. His nephew, James 
Ross, led an expedition to the Antarctic in 1839-43 and collected benthic 
animáis from as deep as 730 m. Such was Forbes’s influence, however, that 
proponents of the azoic hypothesis clung to their beliefs despite increasing 
contrary evidence. The idea that the coid, dark reaches of the ocean could 
not possibly harbour any sort of life was finally refuted in 1860, when a 
submarine cable was brought up for repair from more than 1830 m and was 
found to have encrusting animáis growing on it. Now there was a growing 
Ímpetus to organize deep-sea expeditions to study this vast unknown 
environment and its inhabitants. 

Charles Wyville Thomson became Edward Forbes’s successor as professor 
of natural philosophy at the University of Edinburgh. In 1873, he published 
one of the first texts of oceanography, The Depths of the Sea , based on a 
review of early expeditions. Thomson also became organizer and leader of 
the first oceanographic expedition to circumnavigate the world. This was the 
Challenger Expedition of 1872-76, which travelled 110900 km visiting all 
the major oceans except for the Arctic. The expedition was organized by The 
Royal Society specifically to survey the oceans with respect to their physical 
features, chemistry, and biology. The ship was a British naval sailing vessel 
with auxiliary steam, specially outfitted for scientific work (Figures 1.5 and 
1.6). Besides Thomson, two other naturalists completed the joumey. 

Henry N. Moseley was described as an indefatigable scientist and an 
enthusiastic amateur artist who contributed many drawings to the final 
official report. John Murray, a Canadian-born Scotsman, carried out his 
duties as a naturalist and later was instrumental in publishing the results of 
Figure 1.4 Edward Forbes, a founding father the voyage. The contrasts between the scientists’ viewpoint of the voyage, 
of modern oceanography. and that of the ship’s crew can be seen below. 


Figure 1.5 H.M.S. Challenger. 

‘Strange and beautiful things were brought to us from time to time, 
which seemed to give us a glimpse of the edge of some unfamiliar 
world.’ C. Wyville Thomson, The Challenger Expedition (1876). 

From a naval officer’s diary: ‘Dredging was our bete noire. The 
romance of deep-sea dredging or trawling in the Challenger , when 
repeated several hundred times, was regarded from two points of view; 
the one was the naval officer’s who had to stand for 10 or 12 hours at a 
stretch, carrying on the work ... the other was the naturalist’s ... to 
whom some new worm, coral, or echinoderm is a joy forever, who 

Figure 1.6 The zoological laboratory on board 
H.M.S. Challenger. 


retires to a comfortable cabin to describe with enthusiasm this new 
animal, which we, without much enthusiasm, and with much weariness 
of spirit, to the rumbling tune of the donkey engine only, had dragged 
up for him from the bottom of the sea.’ 

Although a vast amount of information about the ocean had already been 
amassed by the time of the Challenger Expedition, much had been collected 
incidentally or in bits and pieces by individual scientists. The Challenger 
voyage attempted to intégrate biology, chemistry, geology, and physical 
phenomena, and it established systematic data collection using standardized 
methods. For these reasons. the Challenger Expedition is considered to mark 
the beginning of modern oceanography. Over 76 scientists analysed the 
collections made during the voyage, and it took 19 years before all 50 
volumes of final reports were published under the direction and financial 
patronage of John Murray. The expedition produced a basic map of the 
seafloor, and proved without doubt the existence of life at great depths. The 
biological samples yielded 715 new genera and 4417 new species of marine 

Figure 1.7 Some of the 3508 new species of 
radioiarians collected by the Challenger 
Expedition, and described and illustrated by the 
Germán zoologist Ernst Haeckel. 

1 l AñCHlCAPSA , 3*íi HALICAPSA, 7 PLAimjfiSA, 



organisms; 3508 of them were new species of Radiolaria (a group of 
protozoans) (Figure 1.7), all described by the great Germán biologist Ernst 

The monograph on echinoderms was researched and complied by 
Alexander Agassiz of the United States, who said T felt when I got 
through that I never wanted to see another sea urchin and hoped they 
would gradually become extinct.. 

The Challenger Expedition also discovered the true nature of Bathybius 
haeckelii, described by Tilomas Huxley in 1868. Bathybius was a thin layer 
of mucus-like jelly covering the surface of preserved mud samples, and 
Huxley believed that it represented a primordial living slime which carpeted 
the deep seafloor. This ‘organism’, however, proved to be a precipítate of 
calcium sulphate, the result of mixing alcohol with seawater to preserve 
bottom samples. 

‘Never did an expedition cost so little and produce such momentous 
results for human knowledge.’ — Ray Lancaster 

Many other expeditions followed on the path blazed by the Challenger , and 
a few of those that made major contributions to biological oceanography are 
listed in Table 1.2. John Murray went on to organize the Michael Sars 
expedition of 1910, and in 1912 he co-authored a classic text in general 
oceanography, The Depths of the Ocean , with Johan Hjort, a Norwegian 

Table 1.2 Major biological oceanographic expeditions. 




Major objectives/advances 




Global biological 
collections; existence of life 
in deepest waters 




Dredging; Caribbean and 
Gulf of México collections 

Princesse Alice 

I and II; 

Hi ronde lie 

I and II 



Deep-sea collections 




Deep sea; Pacific and Indian 
Ocean collections 




Plankton collections 




Vertical distribution of 
pelagic organisms; deep-sea 

Michael Sars 



Mid- and deep-water 
collections; North Atlantic 

Dana I and II 



Deep-water global 
collections; fishery research 


I and II 



Antarctic ecology 




Atlantic biology 




Deep-sea dredging to 

10000 m; global collections 




Biology of trenches 





Deepest manned dive 
(10916 m, Mariana Trench) 





Discovery of deep-sea hot 


Work was being conducted on marine plankton even before the time of the 
Challenger . The first person reported to have studied marine plankton was a 
surgeon and amateur naturalist, J. Vaughan Thompson, who towed a simple 
fine-meshed net to collect plankton off the coast of Ireland in 1828. His 
studies resulted in the first description of the planktonic stages of crabs. 
Charles Darwin also used a similar net to collect marine plankton during 
his stint as an unpaid, and seasick, naturalist on the voyage of the Beagle, 
from 1831 to 1836. In 1847, Joseph Hooker recognized that the diatoms 
collected in plankton nets were plants, and he suggested that they played the 
same ecological role in the sea as green plants do on land. However, it was 
not until 1887 that the term ‘plankton’ (see Section 1.2) was actually defined 
by Víctor Hensen, a professor at Kiel University who also led the first 
oceanographic expedition entirely devoted to quantitative collections of 
plankton (the Germán ‘Plankton Expedition’ on board the National , see 
Table 1.2) The word ‘plankton’ was more critically defined in 1890 by Ernst 
Haeckel, and today the word encompasses all drifting organisms including 
plants (phytoplankton), animáis (zooplankton), and bacteria 

Monographs on many different groups of zooplankton were available by the 
end of the nineteenth century, and taxonomic guides to phytoplankton were 
beginning to appear. Increasing attention was also given to those organisms 
that were too small to be collected by nets, as indicated below. 

‘... H. H. Gran [a Norwegian scientist] now commenced using his big 
steam centrifuge for centrifuging the water samples from different 
depths, (and) he continued to avail himself of its help until the end of 
the cruise. By means of it he was able to collect in a little drop below 
the microscope all the most minute organisms, and in spite of the 
movements of the little ship and the vibration from the propeller, he 
was able with his microscope to study the many hitherto unknown 
forms in their living State, to draw them, and to count the number of the 
different species.’ — John Murray’s account of a cruise on Michael 
Sars, as related in The Depths of the Ocean (1912). 

The late 1800s and early 1900s also marked the establishment of several 
marine and oceanographic laboratories, many of which were founded by 
biologists. In Europe, the Germán zoologist, Antón Dohrn, established the 
Stazione Zoológica de Napoli in 1872; the station was unique at that time in 
that its facilities were available to visiting scientists of other countries. The 
Marine Biological Association of the United Kingdom started a laboratory in 
Plymouth, England, in 1888. In 1906, Prince Albert I of Monaco 
established an oceanographic museum and aquafium to house extensive 
collections made by his research ships (see Table 1.2). In America, Louis 
Agassiz (father of Alexander Agassiz) established the first marine biological 
laboratory on the east coast in 1873; this was later (1888) moved to Woods 
Hole where it became the Marine Biological Laboratory. During this period, 
Spencer Baird started the first of a series of laboratories devoted to fisheries 
studies in Woods Hole and, in 1930, the Woods Hole Oceanographic 
Institution was officially established. On the west coast of America, William 
Ritter (a student of Alexander Agassiz) founded a research organization in 
1905 that eventually (in 1924) became the Scripps Institution of 
Oceanography in La Jolla, California. Now, almost all countries bordering 
the ocean have oceanographic or fisheries stations. 

The history of biological oceanography is as much intertwined with the 
chemistry of seawater as it is with the nature of marine animáis and plants. 


An understanding of the ecological role of phytoplankton required 
measurements of nutrients in seawater, and these were first carried out by 
the Germán chemists Brandt (1899) and Raben (1905). Later, the English 
chemist H. W. Harvey extended the earlier work of measuring nitrates and 
phosphates to inelude other nutrients such as iron and manganese. 

The beginning of an ecological understanding of the sea carne from the first 
textbooks that attempted to intégrate biological data with the physical and 
Chemical properties of the sea. One of the earliest and most comprehensive 
texts in this respect was The Oceans (1942) by Sverdrup, Johnson, and 
Fleming. Later ecological works included the 1957 publication of Volume 1, 
Ecology , of the Treatise on Marine Ecology and Paleoecology edited by 
J. W. Hedgpeth; G. Riley’s Theory of Food Chain Relations in the Oceans 
(1963); and the delightfully written and illustrated text by Alister Hardy, 
titled The Open Sea: Its Natural History (1965). 

Alister Hardy was a wonderful storyteller, equally at home describing 
his hot-air ballooning experiences in England, or relating shipboard 
experiences like the following from a voyage to Antarctic waters on 
board Discovery I. ‘Expecting a host of surface life we slung a bo’sun’s 
chair (a board supported by ropes on each side like a swing) cióse to 
the water ... right in front of the bows themselves. Here Kemp and I 
took turns with a hand-net and bucket. For sheer pleasure it was ideal: 
swinging in mid-air and gently rising and falling with the swell over the 
deep blue surface which occasionally rose to bathe and cool one’s legs; 
one advanced like a gliding and soaring bird with nothing in front of 
one but the virgin ocean, as yet quite undisturbed by the bows behind 
... I rodé in triumph, fishing out treasure after treasure as they carne 
floating towards me on the very gentle undulating swell. An experience 
never to be forgotten/ From Great Waters (1967). 

One unfortunate development in the history of biological oceanography was 
that much of the study of the top marine predators, the fish, fell under a 
sepárate discipline, that of fishery Sciences. This carne about because the 
most abundant marine fish form the basis of commercial fisheries. The 
branch of fishery Sciences was founded in about 1890, led by Alexander 
Agassiz in the United States, Frank Buckland in England, and 
W. C. Mclntosh in Scotland, all of whom looked to ocean Science as a 
means of improving fish catches. 

In 1902, the International Council for the Exploration of the Sea (ICES) 
was established under the auspices of King Oscar II of Sweden. This 
organization attempted to intégrate physical studies of the oceans with 
biological investigations of fish, but this was not completely successful. 
Scientists trained in physics or chemistry and those trained in biology used 
different methods and approaches, and they proceeded to work independently 
of each other. ICES proved unable to forcé effective legislation concerning 
control of endangered stocks, or overfishing, and the organization was not as 
innovative in developing new fishing techniques and discovering new 
fisheries stocks as the fishermen themselves. After the war, ICES sponsored 
co-operative expeditions in the North Sea and North Atlantic, paid for by the 
national institutions in each participating country. 

Fisheries management strategies tended to concéntrate on economic models 
based on fish abundance and catch, while ignoring the rest of the biology of 
the sea. Classic texts on fisheries dealt primarily with the effeets of 


harvesting on fish population size (e.g. On the Dynamics of Exploited Fish 
Populations by Beverton and Holt, 1957). Increasing human populations and 
increased demand for food resources have driven commercial fisheries to 
expand their fleets while developing new ways to lócate fish schools and to 
harvest the stocks more efficiently. Diminishing stocks of exploited species 
have alerted fisheries scientists to the appreciation that the abundance of fish 
in the oceans is not only related to the numbers of fish removed, but is also 
greatly influenced by ocean climate. A relatively new field known as 
fisheries oceanography has developed which attempts to relate 
oceanographic data to fluctuations in fish stocks. 

Biological oceanography began as a descriptive Science, and basic 
observations on the biology of marine organisms and their environments 
continué to be an important aspect of biological oceanography. However, the 
development of new techniques and apparatus has changed the scope and 
scale of oceanographic research. Sonar, originally developed during World 
War II to detect enemy submarines, was later employed to study the 
topography of the seafloor, find fish schools, and, most recently, to lócate 
and follow concentrations of larger zooplankton. Submarines and scuba 
diving became more sophisticated, and now both are used to obtain in sita 
observations of marine life. Underwater sound recording with hydrophones 
is now employed to study communication in marine mammals and the 
echo-location of prey by mammals and some fish. Computers have greatly 
decreased the time needed to analyse data routinely, and they are useful 
tools in simulating oceanographic events. The development of satellites and 
remóte sensing has made it possible to map the ocean temperature and to 
trace ocean currents. The scale of research in biological oceanography now 
extends from laboratory studies on the effects of environmental change on 
single phytoplankton cells, to employing satellite imagery to obtain global 
patterns of plant production at the sea surface. 

There is now an awareness that meteorological events in the atmosphere and 
climatic changes in the ocean and on land are connected over vast distances, 
and that humans can also produce impacts on the sea which can be measured 
on a global scale. The latter inelude the air- and water-borne dispersal of 
pesticides and other Chemicals, and the overexploitation of fish and marine 
mammal stocks. As human impacts on the ocean increase, an expanding 
base of knowledge about the ecology of the seas becomes essential to 
address questions of exploitation and pollution with more certainty. 

QUESTlüN 1,2 Why is the history of biological oceanography so recent 
compared with the development of terrestrial biology? 


1 The marine environment provides about 300 times more inhabitable 
space for living organisms than that provided by land and freshwater 
combined. All known phyla of plants and animáis originated in the sea, and 
there are presently more phyla represented in the oceans than on land. 

2 In comparison to life in air, the fluid nature of the ocean provides a 
buoyant environment in which the effects of gravity on living organisms are 
reduced. Because of this, marine organisms do not have to invest energy in 


building large proportions of skeletal material, and they expend 
comparatively little energy in maintaining buoyancy and in locomotion. 

3 Plant growth in the ocean is limited to the near-surface regions because 
light does not penétrate very far in seawater, and it is further limited by the 
low concentrations of essential nutrients (e.g. nitrate and phosphate) that are 
present at these depths. Because almost all life in the sea depends directly or 
indirectly on plants, the total plant production at the surface determines the 
amount of animáis that can be produced. 

4 Vertical gradients in environmental parameters (e.g. light, temperature, 
pressure) establish depth ranges with distinctive environmental 

5 Despite the vast extent of the marine environment, only 2% of the 
human diet comes from marine resources. However, this represents 20% of 
the high-quality animal protein consumed by humans. 

6 The benthic environment encompasses the seafloor, and those species of 
plants and animáis that live on or within the seabed form the benthos. The 
pelagic environment is that of the water column, from the sea surface to the 
waters immediately above the seafloor; inshore waters form the neritic zone, 
and offshore waters form the oceanic región. Plankton and nekton inhabit the 
pelagic environment; the distinction between the two groups of organisms is 
based on relative swimming ability, with nektonic species being stronger 
swimmers that are able to move independently of current direction. 

7 Pelagic organisms can be classified into size categories ranging from 
femtoplankton (viruses) through intermediate sizes to the largest nekton 

8 On an ecological scale, organisms can be considered individually or in 
assemblages that inelude populations of a single species, or communities 
made up of the populations of many interacting species. The highest unit of 
ecological integration is the ecosystem, which encompasses one or more 
communities as well as surrounding environment. 

9 The life history patterns of all species form a continuum that ranges 
between the extremes described by r-selection and Á'-selection. 

Opportunistic species are adapted to live in variable or transitory 
environments by having short life eyeles, production of many young, and 
high dispersal ability; however, these r-selected species have high mortality 
rates and their populations are often of short duration. Á^selected species 
live in stable environments and usually have population densities near the 
carrying capacity of the environment; these equilibrium species typically 
have longer life spans, produce relatively few young, and have 
comparatively low death rates. 

10 Edward Forbes (1815-54) is regarded as the founding father of 
biological oceanography, and the Challenger Expedition of 1872-76 marks 
the beginning of systematic oceanographic studies that intégrate physical 
phenomena, water chemistry, and biology. 

11 New techniques developed in the mid- to late-1900s expanded the scope 
and scale of oceanographic research. These inelude sonar, submarines, scuba 
diving, underwater sound recording, and remóte sensing from satellites, all 
of which are now used to investígate life in the sea. 

Now try thefollowing questions to consolídate your understanding of this 


QUESTIOM 1.3 When díd the firsl vertebrales appear in the ucean? Refer lo 
ihe Geologic Time Scale in Appendix 1. 

QUESTIOH 1.4 What are some eharacteiisuc features of the environmem ai 
3(XX) m depth in the water column? 

QUESTION 1.5 What is the greaiesi depth reaehed hy a matined diving vessel 
in the oceans? Refer lo Table 1.2. 



In order to understand the ecology of the seas, it is necessary to understand 
the abiotic physical and Chemical constraints of the marine environment to 
which the resident organisms are adapted. Some of these ecological 
constraints derive from the nature of seawater itself, with special properties 
related to the fluid nature of water and to the Chemicals dissolved in this 
fluid. Other abiotic environmental features important to life in the sea result 
from the interplay between the Earth’s atmosphere and the sea surface. 


Sunlight is as essential to life in the sea as it is to life on land. Some fraction 
of the solar radiation penetrating into the sea is absorbed by plants during 
photosynthesis, and this energy is used in the conversión of inorganic matter 

Figure 2.1 A schematic illustration of the 
passage of solar radiation through the 
atmosphere and sea surface, and the proportion 
of photosynthetically active radiation (PAR) 
available at depth in the sea. 


to organic compounds. Some wavelengths of light are absorbed by water 
molecules and are converted to heat, which establishes the temperature 
regime of the oceans. In addition, light in the sea Controls the máximum 
depth distribution of plants and of some animáis. Vision in animáis is 
dependent on light, and certain physiological rhythms such as migrations and 
breeding periods may be set by periodic light changes. 


Biological oceanographers have tended to use a variety of units to measure 
solar radiation at the sea surface, and to measure light intensity at depth in 
the sea. For that reason, conversions between different units are given in 

Figure 2.2(a) Solar radiation spectra before 
and after passage through the atmosphere, 
showing the zone of PAR (photosynthetically 
active radiation) or visible light. 

(b) Percentage of light reflected from a calm 
sea surface as a function of sun angle. 

(c) Changes in overhead solar radiation received 
at the sea surface with latitude and season in 
the Northern Hemisphere (contour lines 
represent latitude). 


Appendix 2, and these should be referred to when necessary. Two light units 
used for biological studies in the sea are the einstein (E), which measures 
photons (one einstein is a mole of photons, or 6.02 x 10 23 photons), and the 
watt (W), which measures the energy of radiation. The energy of radiation 
depends on the wavelength of the light, but for photosynthetic radiation (400 
to 700 nm), one W m -2 is approximately equal to 4.16 ¡xE m~ 2 s” 1 . 

Solar radiation Corning from the Sun to the outside of the Earth’s atmosphere 
is fairly constant (Figures 2.1 and 2.2a). About half of this energy is 
absorbed and scattered in the various layers of the atmosphere, so that the 
amount reaching the sea surface is about 50% of that received at the top of 
the atmosphere. Some of this is reflected back into the atmosphere from the 
sea surface (Figures 2.1 and 2.2b). The amount reflected depends on the Sun 
angle and becomes very large below a Sun angle of 5 o to the horizon. 

During any day, the actual amount of radiation reaching the sea surface at 
any point is thus a function of the Sun angle, the length of the day, and 
weather conditions. The Sun angle is determined by the time of year, time of 
day, and by the latitude. At the Equator, radiation from an overhead Sun is 
fairly constant throughout the year but, at 50° N, the seasonal variation in 
incident radiation ranges from about 1000 fiE m -2 s -1 in January to over 
4000 fiE rrT 2 s -1 in June (Figure 2.2c). 

QUE SISON 2.1 What is the approximate máximum solar radiation received at 
the surface oí the A rabian Sea oíf Bombay (latitude ca* 20 N) in 
(a) Sepiember and (h) January? Refer to Figure 2,2c, 

A summary of temporal variations in radiation at the sea surface is given in 
relative units in Figure 2.3. The diel variation is the change in solar radiation 
over 24 hours (i.e. the difference between day and night). Diurnal variations 
are those that occur during hours of daylight due, for example, to cloud 
cover. Seasonal variations are most marked at high latitudes. This is 
particularly so within the Arctic Circle where there can be 24 hours of 
sunlight on the ocean surface during the summer. Differences in the input of 
surface solar radiation account for much of the difference in photosynthesis 
by phytoplankton at discrete localities in the ocean. 


In comparison with other liquids, water is relatively transparent to solar 
radiation, but much less so than air. Of the sunlight penetrating the sea 
surface, about 50% is composed of wavelengths longer than about 780 nm. 
This infrared radiation is quickly absorbed and converted to heat in the 
upper few metres (Figure 2.1). Ultraviolet radiation (< 380 nm) forms only 
a small fraction of the total radiation, and it also is usually rapidly scattered 
and absorbed, except in the very clearest ocean waters (Figure 2.4). The 
remaining 50% of the radiation comprises the visible spectrum, with 
wavelengths of between approximately 400 and 700 nm that penétrate 
deeper in the sea. These are of particular importance for animáis with visión, 
and because they are also approximately the same wavelengths used by 
plants in photosynthesis. These wavelengths are often referred to as 
photosynthetically active radiation (PAR). The máximum intensity of PAR 
with the Sun directly overhead is about 2000 ¡jlE m -2 s -1 . Obviously this 
valué will vary with Sun angle, and it decreases to zero as the Sun 
approaches the horizon. 


Figure 2.3 Temporal variations in surface 
solar radiation. (Relative scales). 






Figure 2.4 The penetration of light of different 
wavelengths into clear oceanic water. The lines 
indícate the depths of penetration for 10% and 
1% of the surface light levels. 

wavelength (nm) 

300 400 500 600 700 

Latitudinal § 

(equatorand 3 

Arctic summer) « 


As light passes through water, it is both scattered and absorbed, with 
different wavelengths of the visible spectrum penetrating to different depths 
(Figure 2.4). Red light ( ca . 650 nm) is quickly absorbed, with only about 
1% still remaining at 10 m in very clear seawater. Blue light (ca. 450 nm) 
penetrates deepest, with about 1% remaining at 150 m in clear water. 

There is an exponential decrease of light intensity with depth. An extinction 
coefficient, k, is calculated to express this attenuation of light. The extinction 
coefficient of seawater can be calculated from measurements taken with a 
radiation meter lowered into the sea, and using the following equation in 
which /o is the surface radiation and Id the radiation at depth: 


k _ logf(/o) ~ log ^ /p) (2 1) 

depth (m) 

CUESTION 2.2 The radiación ai 10 m is 50^ of the surface radiación as 
measured with a radiación meter. What is the extinctíon coefficient? 

As you might have inferred from Figure 2.4, the extinction coefficient, k , is 
different for various wavelengths of light. It is about 0.035 m -1 for blue 
light, but about 0.140 m _1 for red light. However, if many partióles are 
present in the water, the blue light is scattered more than the red, and this 
will affect the colour spectrum of undersea light, resulting in a shift of the 
most deeply-penetrating wavelength toward a green colour (Figure 2.4). The 
extinction coefficient is also affected by the amount of coloured, dissolved, 
organic material in seawater, and by the amount of chlorophyll contained in 
living phytoplankton and in plant debris. In the clearest ocean water of the 
tropics, light which is visually detectable by a deep-sea fish may penétrate to 
more than 1000 m (Figure 2.5). In turbid Coastal waters, scattering and 
absorption of light are increased by the presence of much silt and numerous 
phytoplankton, and the same amount of light may not reach 20 m. 

QUESTION 2,3 Novice scuha di vei s are ofien di sappoinled ai seeing a coral 
reef for ihe firsi time beeause of the monotony of colour compared with 
colour photographs and films of red's. Why is this so? 

Three vertical ecological zones in the water column are defined by the 
relative penetration of light in the sea (Figure 2.5). The shallowest zone is 
called the euphotic zone, and it is defined as that región in which light is 
sufficient to support the growth and reproduction of plants. Here, there is 
sufficient light for plant production by photosynthesis to exceed the loss of 
material that takes place through plant respiration (see Section 3.2). The 

Figure 2.5 The vertical ecological zones 
established by light penetration in the sea. Note 
that the light intensity scale is logarithmic with 
depth. The positions of the vertical broken lines 
delimiting the three ecological light zones are 
approximate only (see text). 

light intensity (pWcm 2 ) 


amount of light required for photosynthetic production to just balance 
respiratory losses in plants is known as the compensation light intensity. 
The depth at which photosynthetic production is balanced by plant respiration 
is called the compensation depth, and it defines the lower boundary of the 
euphotic zone. Thus the euphotic zone extends from the surface to a depth of 
just a few metres in turbid inshore regions, to a máximum depth of about 
150 m in very clear, tropical oceanic water. In any región, the compensation 
depth (De), and thus the depth of the euphotic zone, can be calculated from: 

Dc = 'og,('o>- U> g,('c) (2 . 2) 


Surface radiation (/o) is measured directly, and k is calculated from 
equation 2.1 assuming a wavelength of 550 nm. The valué used for the 
compensation light intensity (le) varíes with different species of 
phytoplankton as well as with the previous history of light adaptation of any 
particular species. For example, heavily shaded phytoplankton can adapt to 
lower compensation light intensities. In general, however, valúes for le will 
range between 1 and 10 (i E m~ 2 s -1 . 

Below the euphotic zone is the dimly lighted disphotic zone, a región where 
fish and some invertebrates can see, but where light is too low for positive 
net photosynthesis (i.e. loss of plant material through respiration exceeds 
plant production by photosynthesis over 24 hours). However, living 
phytoplankton which have sunk from the euphotic zone may be present here. 

The deepest and largest región in the open ocean is the dark aphotic zone; 
this extends from below the disphotic zone to the seafloor. Here, sunlight 
cannot be detected by any biological System. This vast región does not 
support plant Ufe, and is spatially removed from the initial link in the marine 
food chain. 

QUESTION 2,4 What do yon ihink is the biológica] role of moonlighl (see 
Figure 2.5) in the sea? 


Water temperature is one of the most important physical properties of the 
marine environment as it exerts an influence on many physical, Chemical, 
geochemical, and biological events. Temperature Controls the rates at which 
Chemical reactions and biological processes (such as metabolism and growth) 
take place. Temperature and salinity variations combine to determine the 
density of seawater, which in turn greatly influences vertical water 
movements with consequent changes in Chemical and biological events 
within the water column. Water temperature partly determines the 
concentration of dissolved gases in seawater; these inelude oxygen and 
carbón dioxide, which are profoundly linked with biological processes. 
Temperature is also one of the most important abiotic factors influencing the 
distribution of marine species. 


There is a continuous exchange of heat and water between the ocean and 
atmosphere. The seas are heated primarily by the infrared wavelengths of 



Figure 2.6 The global distribution of sea surface temperatures (°C) in (a) February and 
(b) August. 

solar radiation. The radiant energy of these wavelengths is quickly 
transformed into heat by absorption. The heating effect of sunlight is 
confined to the immediate surface of the ocean, with 98% of the infrared 
spectrum being absorbed within about the first metre of the water column. 

Sea surface temperatures vary with latitude (Figure 2.6). Surface 
temperatures can exceed 30°C in the tropical open ocean, and approach 40°C 
in shallow tropical lagoons. At the other extreme, water surface temperatures 
in polar regions may be as low as — 1.9°C the freezing point of typical 
seawater. The modérate regime of surface seawater temperature is in sharp 
contrast to air temperatures affecting terrestrial ecosystems; these range from 
as high as 58°C (in northern Africa during summer) to —89°C in the 
Antarctic during winter (Figure 2.7). The temperature regime of the oceans 
is buffered by certain physical properties of water. Water has a very high 
specific heat, meaning that it can absorb or lose large quantities of heat with 
little change in temperature. Furthermore, the oceans are cooled primarily by 
evaporation and, because the latent heat of evaporation for water is the 
highest of all substances, great quantities of heat can be transferred and 
stored in water vapour with relatively little change in water temperature. 

It is sometimes convenient to designate biogeographic zones based on sea 
surface temperatures. The following zones lie within the boundaries set by 
the annual average surface temperatures given in the right column: 









5°C (northern limit) 
2°C (Southern limit) 
< 0-2° or 5°C 

Températe zones in both hemispheres are characterized by a mixture of 
subpolar and subtropical water, and by having the maximal annual 
temperature range. Although attempts have been made to ascribe latitudinal 
limits to these temperature zones, this has little or no ecological significance 
in the pelagic environment where currents displace water of different 
temperatures away from their origins, and where water temperature changes 
very gradually through mixing. In pelagic communities, faunal boundaries 
follow certain isotherms (lines of equal temperature), or are more precisely 
described by combinations of temperature and salinity which define 
distinctive bodies of water (see Section 2.4). 

The mean daily variation in surface temperature in the open ocean is very 
small, generally less than 0.3°C, and it is usually imperceptible at 10 m 
depth. Even in shallow water, the daily surface temperature change is less 
than 2 o C. Temperature changes o ver 24-hour periods are therefore of little 
importance to plankton and fish, unlike residents of intertidal and terrestrial 
ecosystems which can be subjected to very considerable differences between 
day and nighttime temperatures. 

Annual surface temperature fluctuations (compare Figures 2.6a and b) are 
very small in Antarctic waters and are less than 2-5°C in Arctic and tropical 


highest recordad aír 
(Libya, 1922) 

average surface 
temperature near 

lower observed limit, 
breeding range of 
Emperor penguín 
(-18 to -62) 




surface maxíma in 
shallow Coastal 

average máximum 
surface temperature, 
Red Sea 

average mínimum 
22 "\ for survival of 
reef coráis 











*1.33 average 
Antarctic surface 

1.65 mínimum 
\ Antarctic bottom 
*1.87 freezíng point, 
at atmospheric 

lo west recordad air 
(Antárctica, 1983) 

Figure 2.7 Temperature ranges in the sea 
(blue) and on land (black). 

seas. In températe and subtropical areas, they are large enough to influence 
biological events significantly. In the open ocean at latitudes of 30-40°, 
where clear skies permit the maximal heat gain in summer and maximal heat 
loss in winter, the annual variation is about 6-7°C. However, the western 
areas of the North Pacific and North Atlantic have annual variations of up to 
18°C because of the prevailing westerly winds that bring very coid 
continental air masses over these regions in winter and warm continental air 
in summer. In shallow marginal seas, and Coastal areas generally, the 
fluctuation in water temperature closely parallels air temperature, and annual 
variations may exceed 10°C. 

In addition to daily and seasonal variations in surface temperature, there are 
longer term climatic changes that affect marine ecology. Some of these 
events are deduced only from changes in seafloor sediments that suggest 
dramatic temperature changes in the overlying water during the geological 
past. Other climatic changes can be observed in the present time and inelude 
major perturbations such as the El Niño events in the Pacific Ocean. These 
are eyelieal changes in sea surface temperature occurring every two to ten 
years that have widespread impaets on marine ecology as well as on global 
weather. An El Niño can have a catastrophic impact on commercial fisheries 
in affected areas, and the details of this event are considered in Section 6.7.2. 


Turbulent mixing produced by winds and waves transfers heat downward 
from the surface. In low and mid-latitudes, this creates a surface mixed 
layer of water of almost uniform temperature which may be a few metres 
deep to several hundred metres deep (Figure 2.8). Below this mixed layer, at 
depths of 200-300 m in the open ocean, the temperature begins to decrease 
rapidly down to about 1000 m. The water layer within which the 
temperature gradient is steepest is known as the permanent thermocline. 
The temperature difference through this layer may be as large as 20°C. The 
permanent thermocline coincides with a change in water density between the 
warmer low-density surface waters and the underlying coid dense bottom 
waters. The región of rapid density change is known as the pyenoeline, and 
it acts as a barrier to vertical water circulation; thus it also affeets the 
vertical distribution of certain Chemicals which play a role in the biology of 
the seas. The sharp gradients in temperature and density also may act as a 
restriction to vertical movements of animáis. 

Temperature decreases gradually below the permanent thermocline. The 
thermal stratification of the oceans is shown schematically in Figure 2.9. ín 
most oceanic areas, the water temperature at 2000-3000 m never rises above 
4°C regardless of latitude. At greater depths, the temperature declines to 
between about 0°C and 3°C. The temperature of deep water at the Equator is 
within a few degrees of that of deep water in polar regions. The only 
exceptions to coid deep conditions are found in certain localized areas of the 
deep sea, where bottom water temperature may be elevated by geothermal 
activity (see Section 8.9) 

In températe climates, seasonal thermoclines (Figure 2.8) are established in 
the surface layer during the summer. These result from increased solar 
radiation that elevates surface temperature at a time when winds are lessened. 
Thus there is little turbulent mixing to promote downward movement of 
heat, and a thermal stratification is set up in the near-surface waters. This 
phenomenon persists until autumn, when the surface water is cooled 


temperature ( Ú C) 

5 10 15 20 

Figure 2.8 A generalized temperature protile 
for températe latitudes. The solid line shows the 
winter condition with a mixed surface layer of 
homogeneous temperature overlying the 
permanent thermocline. The dashed lines show 
the formation of seasonal thermoclines that 
develop in the surface water in spring and 
summer due to elevated solar radiation and 
warming coinciding with lessened wind. 

and increasing winds cause sufficient turbulence to mix the upper layers and 
break down the thermocline. Beeause permanent and seasonal thermoclines 
greatly affect biological productivity on a global and temporal scale, 
respectively, more will be said about thermal stratification in later sections. 

Jo what way does se a water temperatura affect faunal distribuí i ons? 

The physiological ability to cope with environmental temperatures plays a 
large role in determining the distributional limits of marine organisms. The 
majority of marine animáis (i.e. invertebrates and fish) are poikilothermic 
species with a varying body temperature that approximately follows the 
ambient water temperature, but marine mammals are homoiothermic and 
maintain a constant body temperature. Animáis that can exist in 
environments with a wide temperature range are known as eurythermic. 
Such species tend to have wide distributional ranges or they live in regions 
of considerable temperature fluctuation, such as températe intertidal zones. 
Those species that are restricted to narrow temperature limits are called 
stenothermic. They inelude such groups as reef-building coráis, which 
require a minimum temperature of 20°C, as well as those species that are 
restricted to coid waters. The geographic range of cold-stenothermic species 
may be very wide; for example, some species that are found at shallow 
depths in the Arctic are also present at depths of 2000-3000 m in Equatorial 
areas where similar coid temperatures prevail. 


Salinity refers to the salí contení of seawater. For our purposes, salinity can 
be simply defined as the total weight (in grammes) of inorganic salís 
dissolved in 1 kg of seawater. However, salinity is not measured by weight 
beeause it is difficult and tedious to dry all the salts in seawater. Salinity is 
more easily and routinely determined with a salinometer that measures 
electrical conductivity, which increases with increasing salt contení. The 
major elements are present in the form of ions, with sodium and chloride 
predominating. The ten major constituents listed in Table 2.1 make up about 
99.99% of all the dissolved substances in the ocean. 

Figure 2.9 A generalized and schematic 
cross-section, showing the main thermal layers 
of the oceans and their average temperatures at 
the Equator. 


Why does thís list not inelude such comnion and biológica! I y importa ni 
elements as oxygen, nitrogen, and i ron? 

Other elements and compounds in the oceans are present at lower 
concentrations than those listed in Table 2.1. Some of these, like oxygen and 
carbón dioxide, exist as dissolved gases and will be considered later. Those 
elements that are linked with biological processes (e.g. nitrogen in the form 
of nitrate) exhibit highly variable concentrations, unlike the ions listed in the 
table. Although there also are dissolved organic compounds in seawater, all 
are in concentrations too low to affect salinity. 

Table 2.1 The major constituents of seawater with a salinity of 35. 


(g kg -1 ) 

% by weight 
of all salts 
in the sea 

Chloride (Cl~) 



Sodium (Na + ) 



Sulphate (S0 4 2- ) 



Magnesium (Mg 2+ ) 



Calcium (Ca 2+ ) 



Potassium (K + ) 



Bicarbonate (HCO^) 



Bromide (Br~) 



Borate (mainly H3BO3) 



Strontium (Sr 2+ ) 



Because the concentrations of most of the major constituents are not 
significantly affected by biological and Chemical reactions, they are said to 
show conservative behaviour. This property results in the constancy of 
composition of seawater. That is, the total salinity may vary, but the relative 
proportion of each major ion to the total remains stable, as do the ratios of 
the concentrations of each major ion to the others. These ionic ratios depart 
from normal only in localized regions such as estuaries, which receive an 
inflow of freshwater containing different relative proportions of major ions. 


Variability in salinity is linked with global climate. Salinity in surface waters 
is increased by the removal of water through evaporation, and it is decreased 
primarily through the addition of freshwater via precipitation, either in the 
form of rain or snow, or from river inflow. At higher latitudes, salinity also 
is decreased by ice and snow melt. 

The average salinity of the oceans is about 35, and variability in the global 
distribution of surface salinity in the open ocean is shown in Figure 2.10. 
Salinity valúes closely follow the curve for evaporation minus precipitation 
shown in Figure 2.11. Note that the highest salinity valúes are found at about 
20-30° latitude in both hemispheres, in areas having high evaporation and 
low precipitation. Low salinities are found in polar areas, which have high 
precipitation as well as melting ice, and in areas influenced by polar water. 

Certain marine areas have salinities outside the range of those in the open 
ocean. These generally occur in inshore and shallow areas that are exposed 
to Coastal runoff or river inflow, or that have limited mixing with the open 

Figure 2.10 The mean annual global distribution of surface salinity. The lines connecting 
points of equal salinity are isohalines. 

sea. The salinity ranges given below roughly characterize particular types of 
marine environments: 

Open ocean 
Shallow Coastal areas 

Semi-enclosed seas 
(e.g. Baltic Sea) 

Hypersaline environments 
(e.g. Red Sea; tropical Coastal 

The range of salinity in surface waters is much greater than that in deeper 
layers because fluctuations result primarily from sea surface-atmosphere 
interactions. Figure 2.12 displays the distribution of salinity with depth in 
the Atlantic Ocean. An area where salinity changes rapidly with depth is 
called a halocline. Such zones exist in low and mid-latitudes and lie from 
the bottom of the mixed layer to about 1000 m. Below this depth, salinity is 
34.5-35.0 at all latitudes. 

Diurnal variations in salinity are usually very small, apart from intertidal 
areas or shallow lagoons where evaporation and precipitation effects may be 
intense. Seasonal variations in salinity are also very small, except in inshore 
shallow waters. The average annual variation in surface salinity of the open 
ocean is about 0.3. 

32-38 (average, 35) 


brackish water 

> 40 

E-P (cm) 


Figure 2.11 The distribution of average surface salinity (S, black line) piotted against the 
difference between average annual evaporation and precipitation (E - P, blue line) at different 


In most marine invertebrates and primitive fish (sharks, rays), the salt 
content of the blood and body fluids is about the same as in seawater of 
average salinity. In bony fish (teleosts), the salt concentration of the blood is 
only about 30-50% of the ambient salinity. This has several physiological 
consequences. Because there is a tendency for water to move across 
semipermeable membranes from a zone of Iow salt concentration to one of 
high concentration (a process called osmosis), marine teleost fish tend to 
lose water and thus increase their internal salt concentration. These animáis 
have evolved various physiological mechanisms of osmoregulation that 
counteract this problem. Most marine fish, for example, excrete very small 
quantities of uriñe and secrete salts across the gills. This type of active 
transport, in which the kidneys work against the normal osmotic trend, 
requires an expenditure of energy. Sea turtles, seabirds, and marine 
mammals also exhibit various means of maintaining osmotic balance with 
their environment. 


Figure 2.12(a) A cross-section of the western 
Atlantic Ocean illustrating the vertical 
distribution of salinity. This general pattern is 
typical of all oceans, although the details will 
vary from ocean to ocean. 

(b) Salinity-depth profiles along lines A and B 
in (a). 

The problem of osmotic balance is particularly acute in those marine 
invertebrates and vertebrates that inhabit estuarine areas with rapidly 
changing salinities (see Section 8.5), and in those fish that migrate between 
freshwater and seawater (see Section 6.6.1). Species that can tolérate a wide 
range of salinity are called euryhaline, and they may display various means 
of osmotic control ranging from simple impermeability (for example, closing 
of mollusc shells) to complex forms of active transpon as described above. 
Those animáis that can only tolérate a narrow salinity range are called 

QUESTION 2.5 How could the ambient salinity affect the buoyancy of pelagic 



The density (mass per unit volume) of seawater is governed by temperature 
and salinity (and, to a lesser extern, by hydrostatic pressure). As salinity 
increases, the density increases; as the temperatura increases, the density 
decreases. Salinity and temperatura are physically independent variables but, 
as we have seen, they are not randomly distributed in the ocean. Global 
climate establishes the temperatura and salinity distribution in the surface 
layers of the ocean. Distinctive combinations of these variables are thus 
developed in large volumes of water, and these easily measured 
temperatura-salinity characteristics can be used to define particular water 
masses. Each of these bodies of water thus forms a different type of 
environment, and each supports distinctive communities of organisms. 

Figure 2.13 shows the major upper-layer water masses of the world that 
extend in depth to about the base of the thermocline. The definitive 
temperatura and salinity characteristics of these water masses are acquired at 
the surface, but once water is out of contad with the atmosphere, its 
physical characteristics will only change very gradually and very slowly 
through mixing with adjacent waters of different characteristics. This means 
that even though water masses move both horizontally and vertically in the 
ocean, each can be traced over long distances by its definitive combination 
of temperatura and salinity. New water masses also are eventually formed by 
mixing of waters of different origins, and these too develop their own 
temperatura-salinity signaturas that indicate the amount of mixing. 

The upper water layers are moved horizontally by surface currents generated 
by wind systems (see Section 2.6). Vertical movements of water are 
controlled in part by temperatura and salinity variations that change the 
density of seawater. Figure 2.14 illustrates the relationship between 
temperatura, salinity, and density. Note that density itself cannot be used to 
define a water mass because different combinations of temperatura and 
salinity may produce the same density. 

Figure 2.13 The global distribution of major upper water masses. 

East N. Pacific 
Transitan Waier 

' Atlantic 
Water A 

East N Pac¡k 
Central Water 

Wesl N. Pacific 
Central Water 

Pacific Equatonai Water 

S. Atlantic 
Ceñirá! Waier 


Upper Water 

Wesl S. Pacific 
t Central Water 


j Suban tárete 5 

—¡¡¡¡tfCtlC Surfe^ 


Atlantic Subarclic 
Upper Waier^ 

East S Pacific 
Transffion Water 


QUiSTIÜN 2*6 (aj Whai is i he densiiy of sea water having a temperature oí 
9 C and a salinity of 33*5? (b) What is the densiiy of seawater wiih a 
temperature of 20 C and a salinity of 36,5? Refer to Figure 2.14. 

Water that is less dense than underlying layers will remain at the surface 
(e.g. water in equatorial latitudes). Surface water masses that increase in 
density will sink to depths determined by their densities relative to the 
vertical density structure of the surrounding waters. Figure 2.15 shows the 
water masses lying between about 550 m and 1500 m depth; these masses 
are denser than the waters above 500 m. The densest water masses occupy 
depths from below 1500 m to the seafloor. 

Salinity also has the important effects of lowering the temperature at which 
máximum density occurs, and depressing the freezing point of seawater. 
These changes are shown in Figure 2.16; note that the temperature of 
máximum density and the freezing point are the same at a salinity of about 
25. As the oceans are generally more saline than this (average salinity = 35), 
the density of seawater continúes to increase with decreasing temperature all 
the way to the freezing point (about — 1.9°C at a salinity of 35). In contrast, 
the temperature of máximum density of freshwater (salinity = 0) is 4°C, and 
water becomes less dense as the temperature falls to 0°C, its freezing point. 
This is a profound difference between freshwater and seawater, and it has 
important ramifications on oceanic circulation and on marine life. 

The densest and deepest water masses originate primarily around Antárctica, 
or in the vicinity of Greenland and Iceland (Figure 2.17). During winter in 
high latitudes, surface waters become colder and, because seawater density 
continúes to increase to the freezing point, there is a continual sinking of 
water until that point is reached. As sea-ice forms, it is less saline than the 
seawater so the salinity of the water is elevated, and the density further 
increased. This very dense polar water sinks and flows toward the Equator 
(Figure 2.17) at intermediate depths (Antarctic Intermedíate Water and North 
Atlantic Deep Water) or along the seafloor (Antarctic Bottom Water). 
Antarctic Bottom Water in particular penetrates far into the northem parts of 

Figure 2.14 A T-S diagram showing the 
relationship between temperature (T), salinity 
(S) and density. For convenience, the density 
contours are shown as lines of equal valúes of 
(sigma-t) where a t = (density - 1) x 1000. 
Therefore a density of 1.02781 g crrr 3 has a 
a, = 27.81. 


E. Atlantic Subarctic 
/ I ni Water 

Arctic Intermedíate Water 

/ Water 

^Pacific Subarctic / . jj 
I nte r med ¡ate Water (o rrnan 

^_ I J- 


liate Watnr 


intermedíate W¡ 

f Antarctic 
Intermedíate Water 

Antarctic Intermedíate Water 

Antarctic intermedíate Water 

East S Pacific 
Intermedíate Water 

Figure 2.15 The global distribution of 
intermedíate water masses lyíng between about 
550 and 1500 m depth. The source regions of 
the water masses are indicated by dark blue. 

the Atlantic and Pacific oceans. Deep water will eventually be returned to 
the surface by wind-driven mixing, and thus there is a continuous, but very 
slow (on the order of several hundred to a thousand years), cycling between 
surface and deep waters. 

In températe latitudes during winter, cooling surface water over the deep 
open ocean continúes to sink and never reaches the freezing point. Thus the 
ocean surface remains ice-free, except in shallow marginal seas like the Gulf 
of St. Lawrence off eastem Cañada. By contrast, the water in freshwater 
lakes and ponds may cool to 4°C (its temperature of máximum density) from 
top to bottom. With additional cooling, the surface layer in lakes becomes 
lighter and fioats, and vertical circulation ceases. Then the surface water can 
continué to cool to the freezing point, with ice formation closing the surface 
relatively easily. 

Figure 2.16 Temperatures of freezing and 

máximum density of water as a function of QUESTIÜN 2,7 Huw do voy tfnnk ihe relatively low sahnity ( < 34.5) of 

salinity. Arctic surface water would affect freezing? Kefer to Figure 2.16. 

When dense water sinks from the surface, water moves horizontally into the 
región where sinking is occurring, and elsewhere water rises to complete the 
cycle. The horizontal or vertical movement of water is referred to generally 
as advection. Because water is a fixed quantity in the oceans, it cannot be 
accumulated or removed at given locations without movement of water 
between these regions. The sinking of water is called downwelling; upward 
movements of water are called upwelling. Downwelling transports 
oxygen-rich surface water to depth; upwelling retums essential nutrients (e.g. 
nitrate, phosphate) to the euphotic zone where they can be utilized by plants 
to produce organic materials. Because upwelling is so important for marine 
productivity, it is discussed in more detail in Section 3.5. 


Figure 2.17 A cross-section of the Atlantic Ocean showing the formation and movement of 
intermedíate and deep-water masses from polar regions. AABW, Antarctic Bottom Water; 
AAIW, Antarctic Intermedíate Water; and NADW, North Atlantic Deep Water. 


Figure 2.18 The relationship between pressure 
and depth. Both scales are logarithmic simply 
to accommodate the range of numbers. 

Hydrostatic pressure is another physical environmental factor affecting life 
in the sea. Pressure is determined by the weight of the overlying water 
column per unit area at a particular depth. For the purposes of this book, the 
relationship between pressure and depth is considered to be effectively linear 
although, in fací, pressure is also influenced by density which increases with 

Pressure can be measured in a variety of units. Figure 2.18 expresses 
pressure in newtons per square metre. At 10 m depth, the pressure is 
10 5 N m -2 ; this is roughly equivalent to 1 atm or 1 bar, which are the units 
conventionally used by biologists (and scuba divers). It is easiest to 
remember that, with increasing depth, pressure increases by 1 decibar every 
metre (= 10 4 N m~ 2 ), or by 1 atm every 10 m. 

No matter which units of pressure are used, it is apparent that many marine 
organisms inhabiting deep waters are subject to very high pressures. In the 
deepest ocean basins, organisms exist at pressures exceeding 1000 atm. 
There are also marine animáis that daily migrate vertically over distances of 
several hundred metres and thus experience considerable pressure changes. 

Because of the difficulties in collecting deep-sea species under pressure and 
maintaining thern at their in situ pressures in the laboratory, the biological 
effects of pressure remain somewhat uncertain. It has also been difficult to 
sepárate the effects of hydrostatic pressure on the metabolism of deep-sea 


animáis from those effects due to living in a low temperature and dark 
environment. It is known, however, that liquids can only be slightly 
compressed under high pressure, but that gases are highly compressible. This 
means that animáis with gas-filled structures, like the swim bladders of some 
fish, may be markedly affected by pressure change, whereas animáis lacking 
these structures may be more tolerant of depth change. Air-breathing marine 
mammals, with gas-filled lungs, have evolved a variety of special anatomical 
and physiological adaptations that permit them to make deep dives. More 
generally, experiments have shown that many planktonic organisms are 
sensitive to pressure and will respond to laboratory-controlled pressure 
changes by swimming upward or downward. Animáis living permanently in 
the deep sea do not have gas-filled organs and may have special biochemical 
adaptations to living under high pressures. 

Certain animáis (both benthic and pelagic species) do inhabit wide depth 
ranges in the sea and these are considered to be eurybathic. Other species 
are intolerant of pressure change and remain restricted to narrow depth 
ranges; these are the stenobathic species. Indeed, some stenobathic forms 
are restricted to deep areas and seem to require high pressures for normal 


The major surface currents in the ocean (Figure 2.19) are primarily 
wind-driven and thus closely related to the major wind systems. However, 
the eastward rotation of the Earth modifies the direction of water movement 
by deflecting currents to the right in the Northern Hemisphere, producing a 
tendency to clockwise circulation patterns. In the Southern Hemisphere, the 
deflection is to the left, and major currents move counterclockwise. 

Figure 2.19 illustrates the major clockwise gyres in both the North Atlantic 
and North Pacific oceans. North of the Equator, persistent north-east trade 
winds forcé water westward to form a North Equatorial Current in both 
oceans. When this water reaches continental land masses, it turns northward 
as the Gulf Stream in the Atlantic and as the Kuroshio in the Pacific. At 
about 40°N, dominant westerly winds assist returning eastward-flowing 
currents. The circuits are completed by water fiowing southward to form the 
Canaries Current in the Atlantic, and the California Current in the Pacific. 

The oceanic gyres flow counterclockwise in the Southern Hemisphere, 
forming mirror images of their northern counterparts. South-east trade winds 
generate westward-flowing South Equatorial Currents, most of which are 
deflected southward (left) along eastern South America (Brazil Current) and 
Australia (East Australia Current). Water flows northward along western 
Africa in the Atlantic and along Chile and Perú in the Pacific before 
rejoining the South Equatorial Currents. 

In all of these gyres, currents are narrower, deeper, and faster along the 
western edges of the oceans compared with those along the eastern margins. 
For example, the Gulf Stream and Kuroshio are western boundary currents 
fiowing at velocities up to 200 cm s -1 , roughly ten times faster than eastern 
boundary currents (< 20 cm s" 1 for the Canaries or California currents). As 
these large volumes of water circuíate, they mix with other water bodies and 
their characteristics change gradually. Note, for example, the joining and 


[uatorlal Counter-Current 

ÍEquatorial Curfent 





Current [ V \ 

// ¿ FaWafids_ 
^ J durrgnt — 

r* myi wsr 1 20*1^100 o x^,w° 60 ° 40 °w 20° / / jb° ,20^ 


<>\ a- Alaska / '•rvS ( .V-W ( 


> 'S California G¡j!f %^'^'é^-JS; 

'N. Pacific Curren^ vJk Streanyjjír ir 

- -- -^ \ <\ Flarida ''X * >]V¿^ 


- ^,/ > ) Guinea 

¡c^Gkgürriftolar Current (West'Wind Dri 




Figure2.19 The major surface currents oí the oceans in northern winter. Dashed arrows indícate 
cool currents; solid arrows show warm currents. 

mixture of coid, low-salinity Laborator Current water with the warm, high 
salinity Gulf Stream (Figure 2.19 and Colour Píate 7). 

CUESTION 2*8 Figure 2.19 shows one eastward-flowing surface current ihat is 
unim peded by Land barriers and makes a complete Circuit a round the 
world, connecting the Atlantic, Pacific, and ludían oceans. What is the ñame 
of i his cur re n t a n d whe re i s i t 1 oca t ed? 


The issue of ocean circulation is the subject of physical oceanography, and 
the description of generating forces for surface currents given here is far 
from complete. However, the basic pattern of circulation is presented here 
because water movements greatly influence biological productivity. As 
currents meet and mix, or meet continental land masses or major rivers, or 
move over shallower depths, various types of vertical circulation patterns are 
generated that affect the distribution of nutrients available to 
phytoplankton. Section 3.5 explores the mechanisms that produce these 
geographical differences in production. The patterns of ocean currents also 
influence the geographical distributions of both pelagic and benthic marine 

The oceans form a dynamic, fluid environment moving over the surface of 
the Earth, and this creates one of the most difficult problems for biological 
oceanographers because it is impossible to follow the same population or 


community of pelagic organisms for any appreciable time period. A body of 
water sampled at a specific locality will not be the same body of water 1 
hour later at the same position. Even if a patch of surface water is marked 
by a floating buoy or drogue, the resident animáis in the underlying water 
column will change as they vary their depth positions. By doing so, they 
enter water moving at different speeds and in different directions relative to 
the surface. This is why many biological processes that require sampling over 
longer time spans, such as growth rates of zooplankton, are measured in the 
laboratory using captive animáis, or are inferred using indirect techniques. 


1 The amount of sunlight arriving at the sea surface varies with time of 
day, season, and weather. Approximately 50% of the solar radiation 
penetrating the sea surface is within the visible spectrum (about 
400-700 nm), and these are approximately the same wavelengths used in 
plant photosynthesis. The intensity of photosynthetically active radiation 
(PAR) at the sea surface ranges from zero (in darkness) to about 

2000 fiE irr 2 s ' 1 with the Sun directly overhead. 

2 Different wavelengths of light are absorbed and scattered at different 
depths in water, and they have different extinction coefficients, with red light 
being attenuated most rapidly and blue light penetrating deepest in clear 
water. The depth to which any wavelength penetrates depends partly on the 
amount of suspended particles and chlorophyll in the water. 

3 Three ecological zones have been defined, based on the penetration of 
light in seawater. The euphotic zone is that región where light is sufficient 
for the growth of plants, and it extends from the surface to a máximum of 
about 150 m in the clearest oceanic water. The lower boundary is defined by 
the compensation light depth, where only enough light is present for 
photosynthesis to balance plant respiration over 24 hours. The disphotic zone 
is dimly lighted; there is sufficient light for visión, but too little for plant 
production. The deepest and largest zone is the aphotic zone, a región of 
darkness extending to the seafloor where the only light emanates from the 
bioluminescence of certain animáis. 

4 Infrared wavelengths are absorbed within the first few metres of the sea 
surface and are the primary heat source of the oceans. Sea surface 
temperatures vary with latitude and fluctuate seasonally but remain within a 
modérate range of about 40°C to — 1.9°C, the freezing point of water with a 
salinity of 35. 

5 In many parts of the ocean, there is thermal stratification consisting of an 
upper mixed layer of water of almost homogeneous temperature; a región of 
rapid temperature decrease known as the permanent thermocline; and an 
underlying coid deep layer of water formed originally at the surface in polar 

6 In mid-latitudes where seasons are pronounced, seasonal thermoclines are 
formed in the surface layer during spring and summer. These zones of steep 
temperature change are established because increased solar radiation elevates 
surface temperatures at a time when lessened winds reduce the amount of 
mixing in the water. 


7 The average salinity of the open ocean is about 35 parts per thousand by 
weight, with ten major ions making up about 99.99% of all the dissolved 
substances in the oceans. In inshore or isolated areas with little water 
exchange, salinity may vary from about 5 to 25 in brackish waters, to more 
than 40 in such hypersaline areas as the Red Sea and some shallow lagoons. 
Variations in salinity are primarily caused by evaporation (which elevates 
salinity) and precipitation (which decreases salinity). 

8 Whereas total salinity is variable, the major dissolved ions are not 
significantly affected by biological or Chemical reactions and the relative 
proportions of these dissolved constituents remain constant. 

9 The combined properties of salinity and temperature are used to define 
water masses. Each of these large bodies of water has a discrete origin and 
forms a distinctive environment, supporting a distinctive community of 
pelagic organisms. 

10 Salinity, temperature, and pressure establish the density of seawater. 
Changes at the sea surface that result in higher density will lead to 
downwelling of that water. Very dense water formed at high latitudes sinks 
to form the bottom water masses of the oceans, and this process is important 
in maintaining oxygen levels at all depths. Upwelling of water is partly 
caused by wind-driven mixing and is of importance in returning biologically 
essential elements to surface waters, where they are used by plants in 

11 The salt content of the sea lowers the temperature of máximum density 
and depresses the freezing point of seawater relative to freshwater. This not 
only results in the winter downwelling of polar water (see 10 above), but it 
also prevents sea-ice formation except in polar areas and in shallow 
high-latitude marginal seas. 

12 Oceanic surface currents are generated by global wind systems, and 
their direction is modified by the Earth's rotation. This results in large 
clockwise-moving gyres in the northern oceans and anticlockwise gyres in 
the Southern Hemisphere. The patterns of movement and mixing of these 
currents produces geographic regions of differing biological productivity. 
Horizontal transport of water also establishes the geographic distribution of 
many marine species. 

13 Hydrostatic pressure effectively increases linearly with depth, at a rate 
of 0.1 atm m -1 . In the deepest areas, organisms live at pressures exceeding 
1000 atm. 

Now try the following questions to consolídate your understanding of this 

CUESTION 2J In Seelion 24,2, compensador) light intensities de) for 
phytoplankton are given as ranging between I and 10 /¿E What are 

ihese valúes in watts m : ? tRefer to Appendix 2 for conversión factors.) 

CUESTION 2,10 The majority of marine animáis (boih pelagic and benthic) are 
poikilothertnic, whereas many land animáis (birds, mammais) are 
homoiothermic species. Can yoti think of a reason to explaín this difference? 


QOESTIOH 2.11 Refer lo ihe global ranges of suríace saliniiy shovvn ín 
Figure 2J 1. (a) Explain ihc low salinily valué (34.5) ui ihe Equator. 

(b) Why is salinily higher in surface waters of thc Antarctic (ca. 57 $ 
lalitude) comparecí to ihe Arctic (57 N)7 

QUESTION 2.12 Refer lo Figure 2.14, Whíeh combinaron of high or low 
le ni pe ral u re and high or low saliolty would produce water of grealesl 

QUESTION 2.13 Review whal you have learned about abiolie env ironmemal 
factors ín ibis Chapier and describe the deep-sea environmeni be low 2000 m 
in le mis of light, salinily. tempera ture, pressure. and reí al i ve densiiy. 



The great majority of the plants in the ocean are various types of planktonic, 
unicellular algae, collectively called phytoplankton. Although some 
phytoplankton are large enough to be collected in fine-mesh nets, many of 
these microscopic plants can only be collected by filtering or centrifuging 
sizable volumes of seawater. There are also macroscopic floating algae in 
some oceanic areas, Sargassum in the Sargasso Sea being a well known 
example, but they are relatively restricted in locality. Similarly, the benthic 
species of algae, including attached macroscopic seaweeds, are limited in 
distribution to coastal, shallow areas because of the rapid attenuation of light 
with depth. In contrast, phytoplankton are present throughout the lighted 
regions of all seas, including under ice in polar areas. Because the 
phytoplankton are the dominant plants in the ocean, their role in the marine 
food chain is of paramount importance. 

Table 3.1 A taxonomic survey of the marine phytoplankton. 




Area(s) of 








algae (or 





Red algae 

Coid températe 











Coid waters 




All waters. 



esp. coastal 










Very rare 




Very rare 

Pry mnesiophyceae 














All waters- 




Green algae 





All waters. 



esp. warm 


Pro tope r id i n i u ni 

This table is included for completeness and information, but it is not necessary to 
remember all details. It is important to note the diversity shown among the 



Approximately 4000 species of marine phytoplankton have been described, 
and new species are continually being added to this total. A taxonomic list 
of the major types of phytoplankton is given in Table 3.1, but only the better 
known groups are considered in some detail below. 

3.1.1 DIATOMS 

Diatoms (Figure 3.1; Colour Plates 1 and 2) belong to a class of algae 
called the Bacillariophyceae. They are among the best studied of the 
planktonic algae and are often the dominant phytoplankton in températe and 
high latitudes. Diatoms are unicellular, with cell size ranging from about 
2 ¡im to over 1000 /xm, and some species form larger chains or other forms 
of aggregates in which individual cells are heid together by mucilaginous 
threads or spines. All species have an external skeleton, or frustule, made of 
silica and fundamentally composed of two valves. Silica in the skeleton 

Figure 3.1 Diatoms. (a) a typical Chain of 
Chaetoceros faciniosus, (b) C. iacirtiosus Chain 
with resting spores; (c) Nitzschia pungens Chain 
of dividing cells; (d) Thalassiosira grávida Chain; 
(e) Coscinodiscus showíng the two valves of the 
frustule; (f) Coscinodiscus wailesii, lateral view; 
(g) Chaetoceros sociaiis chains in a gelatinous 
colony formation; (h) a chain of Asterioneila 
japónica, and (i) Skeletonema costatum. (scales 
in mm) 



parent cell 



/ \ 

asexual división 

J— \ 

0 43 

offspring of 
differing size 


diminution of size 



critical minimal size 

sexual reproduction 
and the 

formation of auxospores 
Figure 3.2 The life cycle of diatoms. 

makes up 4-50% of the dry weight of the cell. The frustule is usually 
sculptured into patterns of spines, pores, channels, and/or ribs which are 
distinctive to individual species. Diatoms have been abundant in the seas 
since the Cretaceous (about 100 million years ago) and, over geological 
time, sedimented frustules have formed seafloor deposits called 
diatomaceous ooze. 

Two types of diatoms are recognized: the pennate and centric forms. Pennate 
diatoms have elongate shapes and are mostly benthic, but the few planktonic 
genera such as Nitzschia (Figure 3.1c) may be abundant in some regions. 
Centric diatoms have val ves that are arranged radial ly or concentrical ly 
around a point, and they are much more common in the píankton, with 
somewhat over 1000 species. Chaetoceros , Coscinodiscus, Skeletonema , and 
Thalassiosira are all common centric genera, some of which are illustrated 
in Figure 3.1. 

Planktonic diatoms do not have any locomotor structures and are usually 
incapable of independent movement. Because it is essential for diatoms and 
other phytoplankton to remain in lighted surface waters in order to carry out 
photosynthesis, these algae exhibit a variety of mechanisms which retard 
sinking. These inelude their small size and general morphology, as the ratio 
of cell surface area to volume determines frictional drag in the water. 

Colony or chain formation also increases surface area and slows sinking. 
Most species carry out ionic regulation, in which the internal concentration 
of ions is reduced relative to their concentration in seawater. Diatoms also 
produce and store oil, and this metabolic by-product further reduces cell 
density. In experimental conditions, living cells tend to sink at rates ranging 
from 0 to 30 m day -1 , but dead cells may sink more than twice as fast. In 
nature, turbulence of surface waters is also important in maintaining 
phytoplankton near the surface where they receive abundant sunlight. 

The usual method of reproduction in diatoms is by a simple asexual división 
in which the cell forms two nuclei, the two halves of the frustule sepárate, 
and each resulting daughter cell grows a new inner valve of the frustule 
(Figure 3.2). This can result in the two new cells being of slightly unequal 
size, the one receiving the inner half of the original frustule being slightly 
smaller than the cell formed from the outer valve. Asexual división can lead 
to very rapid population growth under optimal conditions. However, with 
repeated divisions, there may be a diminution in size of some of the progeny. 

When a diatom reaches a certain critical minimal size, it undergoes sexual 
reproduction by forming a cell that lacks a siliceous skeleton and contains 
only half of the genetic material. Such cells fuse to form a zygote, and this 
swells to produce an auxospore. Subsequently, a larger cell is formed that 
ultimately produces a frustule of the normal shape and size. Sexual 
reproduction in diatoms does not necessarily require a reduction in size of 
the cell, however. 

Some diatoms, particularly neritic species living in relatively shallow water, 
produce resting spores (Figure 3.1b) under adverse environmental 
conditions. These form when the protoplasm of a normal cell becomes 
concentrated and surrounded by a hard shell. This heavy spore sinks to the 
bottom and remains dormant until favourable conditions are restored, in 
which case it is capable of becoming a normal planktonic cell. 



The second most abundant phytoplankton group following the diatoms is 
composed of algae belonging to the Pyrrophyceae, and commonly referred to 
as dinoflagellates (Figure 3.3; Colour Píate 3). Most of these unicellular 
algae exist singly; only a few species form chains. Unlike the diatoms, 
dinoflagellates possess two flagella, or whiplike appendages, and are 
therefore motile. 

Different species of dinoflagellates utilize different energy sources. Only 
some dinoflagellates are strictly autotrophic, building organic materials and 
obtaining all their energy from photosynthesis. Other species carry out 
heterotrophic production; that is, they meet their energy needs by feeding 
on phytoplankton and small zooplankton. Indeed, about 50% of the 
dinoflagellates are strict heterotrophs that lack chloroplasts and are incapable 

Figure 3.3 Dinoflagellates. (a) Two views of Prorocentrum marinum, (b) Prorocentrum 
micans, (c) P. micans dividing; (d) Protoperidinium crassipes, (e) Gymnodinium abbreviatu/rr, 
(f) Dinophysis acuta, and (g) Gonyaulax fragilis. (All scale bars represent 0.02 mm.) 



of carrying out photosynthesis; these species form part of the zooplankton 
and are considered in Section 4.2. Some dinoflagellates are mixotrophic, 
and are capable of both autotrophic and heterotrophic production (see also 
Section 4.2). Still other dinoflagellate species are parasitic or symbiotic (e.g. 
see Section 8.6.4). There are an estimated 1500 to 1800 species of 
free-living, planktonic dinoflagellates. 

Conventionally, dinoflagellates are divided into thecate species, which have a 
relatively thick cellulose cell wall called a theca, and naked forms which 
lack this structure. Taxonomically, the dinoflagellates are separated into the 
Desmophyceae and the Dinophyceae. The former is a small group in which 
the species are characterized by having both flagella arising from the anterior 
end of the cell (Figure 3.3a, b). The cell wall is composed of two 
longitudinal valves that sepárate during asexual división to form two new 
cells of equal size (Figure 3.3c). Prorocentrum is a common planktonic 
genus belonging to the Desmophyceae. 

The majority of planktonic dinoflagellate species form the Dinophyceae 
(Figure 3.3d-g; Colour Píate 3), and the majority of these are thecate. In all 
of them, the cell is divided into an anterior and posterior half by a transverse 
groove known as a girdle. The flagella are so arranged that one extends 
posteriorly from the cell, and the other wraps transversely around the cell in 
the girdle región. In those species with a theca, the cell wall is divided into a 
number of sepárate cellulose plates that are ornamented with pores and/or 
small spines. Common thecate genera inelude Ceratium , Protoperidinium , 
Gonyaulax , and Dinophysis. Gymnodinium is a common naked form 
belonging to the Dinophyceae. 

Reproduction in dinoflagellates is normally by simple asexual división, with 
the cell dividing obliquely to form two cells of equal size. The theca may 
divide, with each new cell forming a new half, or the theca may be lost 
before división, in which case each new cell forms an entirely new cell wall. 
Asexual división can lead to rapid population development when conditions 
favour these algae. Dinoflagellates often become abundant in summer or 
autumn, following blooms of diatoms, as they are better adapted at living 
under lower light conditions and in nutrient-impoverished water. This is 
partly because dinoflagellates are capable of moving vertically in the water 
column; during the day they can carry out photosynthesis in sunlit surface 
waters that have been stripped of nutrients by fast-growing algae, and at 
night they may move deeper to take advantage of higher nutrient 
concentrations. For the same reason, dinoflagellates are usually the most 
numerous of the phytoplankton in stratified, nutrient-poor tropical and 
subtropical waters (see Section 2.2.2 and Figure 3.9). 

Sexual reproduction also occurs in at least some species of dinoflagellates. 
This may lead to the formation of thick-walled, dormant cysts that settle on 
the seafloor, where they can survive for years. When triggered by 
environmental change, the cysts germinate to produce swimming cells. 

Phytoplankton blooms develop when a species suddenly increases greatly 
in numbers under favourable conditions. In some circumstances, the rapid 
reproduction of dinoflagellates results in such high densities of organisms 
that their reddish-brown pigment visibly colours the water, producing 
so-called red tides (Colour Píate 4) (see also Section 3.1.3, Cyanophyceae). 
This red water may be caused by very high concentrations of innocuous 
species of dinoflagellates, or of species that contain potent toxins (see 


below). In any case, red tides begin with a sudden increase in numbers of 
the dinoflagellate. The water becomes noticeably coloured when 
concentrations reach about 200000 to 500000 cells 1 _I and, as the bloom 
develops, concentrations may exceed 10 8 cells l~ l . When essential nutrients 
are exhausted by the dinoflagellates and the bloom decays, the bacterial 
decomposition of large amounts of organic material depletes the available 
oxygen and fish may die as a result of the lowered oxygen concentrations. 
The development of anoxic conditions is not exclusively a property of 
dinoflagellate blooms; such conditions can also occur following large blooms 
of other types of phytoplankton. 

Some red tides are caused by certain species of Alexandrium , Pyrodinium , 
and Gymnodinium which produce a variety of neurotoxins collectively 
referred to as saxitoxin, which is 50 times more lethal than strychnine and 
10000 times more deadly than cyanide. Even when present in concentrations 
too low to colour the water, these dinoflagellates can be poisonous to certain 
animáis and to humans. While the dinoflagellates are growing and 
reproducing, they build up saxitoxin in their cells and some of this is 
released into the water. The toxic dinoflagellates are also ingested by some 
zooplankton and by filter-feeding shellfish like clams, mussels, scallops, and 
oysters. Zooplankton and shellfish accumulate and concéntrate saxitoxin in 
their own tissues, where it may be retained for considerable periods without 
harmful effects. However, vertebrates, such as fish, are sensitive to saxitoxin 
and may die from eating contaminated zooplankton. In serious outbreaks, 
seabirds and even dolphins and whales may also perish by accumulating 
saxitoxin from their food. 

The minimum lethal dose of saxitoxin for humans is 7 to 16 ¡xg kg -1 of 
body weight, and eating a single contaminated clam may be enough to cause 
death from paralytic shellfish poisoning (or PSP). Saxitoxin is heat stable, 
so cooking of the shellfish does not destroy the potency of this neurotoxin. 

In North America within historie medical times, about 1000 cases of shellfish 
poisoning have been recorded, with about one-quarter of these resulting in 
death. One of the earliest recorded cases occurred off the west coast of 
Cañada on 15 June 1793, when one death and four illnesses resulted from 
crew on Captain George Vancouver’s ship eating toxic mussels. In 1799, 100 
men on a Russian expedition off Alaska died from eating mussels. Paralytic 
shellfish poisoning remains a problem on both coasts of North America, in 
Central America, and the Philippines; it also occurs in Europe, Australia, 
South Africa, and Japan. In 1987, three human fatalities and 105 cases of 
acute poisoning were reported in eastern Cañada as the result of consuming 
toxic mussels. In this case, the neurotoxic compound was identified as 
domoic acid that originated in a diatom, Pseudonitzschia , which had 
previously been considered harmless. Developed countries typically have 
monitoring programmes that permit the closure of contaminated shellfish 
beds (natural or cultivated) when toxins are detected in the water or in 
shellfish tissues; the incidence of sickness and fatalities from algal-derived 
shellfish poisoning is higher in Coastal developing countries. 

A related health problem, ciguatera fish poisoning (or CFP) is found in 
tropical and subtropical countries, where certain species of toxic 
dinoflagellates live attached to seaweeds. Fish that feed on seaweeds also 
ingest the dinoflagellates and accumulate toxin in their tissues, and this is 
passed on through the food web to carnivorous fish, and eventually to 
humans who consume contaminated fish. Symptoms range from headache 


and nausea in mild cases to convulsions, paralysis, and even death in severe 
cases. It is estimated that CFP causes more human illness than any other 
kind of toxicity originating in seafood, with 10000 to 50000 individuáis 
being affected each year. 


Coccolithophorids (Colour Píate 5) are unicellular phytoplankton that form 
part of the nanoplankton (refer to Figure 1.2), with most of the 150 or so 
species being smaller than 20 ¡jl m. Their outstanding characteristic is an 
external shell composed of a large number of calcareous plates called 
coccoliths. The shape and arrangement of the coccoliths can be used to 
identify species. The coccoliths accumulate in bottom sediments, and they 
are the major constituent of the uplifted sediments known as chalk, which 
forms the famous White Cliffs of Dover. Like the dinoflagellates, 
coccolithophorids possess two flagella, although they may have a life cycle 
which ineludes an alternation with a non-motile stage lacking flagella. 
Although coccolithophorids can be found in neritic as well as in oceanic 
waters (see Figure 1.1), and at times are near the surface, the majority of 
species occur in warmer seas and thrive in reduced light intensities; some 
species reach máximum abundance at depths of about 100 m in clear, 
tropical, oceanic water. However, Emiliania huxleyi is probably the most 
widespread coccolithophorid in the sea, and it is present in all oceans except 
the polar seas. Emiliania sometimes forms enormous blooms, one having 
been measured to cover approximately 1000 km by 500 km of sea surface in 
the North Atlantic Ocean — or an area roughly the size of Great Britain. 
Reproduction in coccolithophorids is by longitudinal división, with the shell 
being divided and afterwards reformed into a whole by each new cell. 
However, life histories in this group are complex and may involve several 
different types of stages. 

Allied with coccolithophorids in the algal group Prymnesiophyceae are 
several other important phytoplankton which lack coccoliths and are 
superficially very different in appearance. These inelude such unicellular and 
motile genera as Isochrysis , a small alga commonly cultured in the 
laboratory, and Phaeocystis , which forms large gelatinous colonies that can 
foul fish nets and also beaches when washed ashore. Prymnesium is 
characteristic of low salinity water and can be a major cause of mortality in 
farmed salmón along the Norwegian coast because it interferes with gas 
exchange across the gills of the fish. 

The best known marine forms of the Chrysophyceae, or golden-brown algae, 
are the silicoflagellates with an internal skeleton formed of siliceous spicules 
(Colour Píate 6). These uniflagellate organisms are small (10-250 /xm) and 
contain very numerous yellow-brown chloroplasts. Only a few species of 
silicoflagellates are known, and these are usually most abundant in colder 

Numerous species of small, naked, flagellated phytoplankton also make up 
other taxonomic divisions (Table 3.1). Some of these species are truly rare, 
but many remain poorly known because of the difficulties in collecting and 
preserving very small cells (including picoplankton of 0.2-2 /xm) which do 
not have rigid skeletal structures. Some flagellates that survive collection 
will disintegrate during filtration or when placed in preservatives. 
Nevertheless, some of these minute phytoplankton can be very abundant and 
important in ecological eyeles. 


Some of the smallest, and also some of the largest, species of phytoplankton 
belong to the Cyanophyceae or Cyanobacteria (also known as blue-green 
algae, or blue-green bacteria). A single genus, Oscillatoria (formerly called 
Trichodesmium) is well known, and -is important in the tropical open ocean. 
At times this alga exists in single long filaments formed by chains of cells; 
at other times, the filaments clump together to form macroscopic bundles of 
several millimetres in diameter. Interest has been directed toward this genus 
as the species are capable of utilizing and fixing dissolved gaseous nitrogen 
(N 2 ), unlike other phytoplankton which can only utilize combined forms of 
nitrogen such as nitrate, nitrite, and ammonia. The attribute of nitrogen 
fixation may explain the relative success of Oscillatoria in tropical waters 
which typically have low concentrations of the nitrogen sources normally 
utilized by other algae. Nitrogen fixation does not seem to be a physiological 
feature of Synechococcus , another genus of marine cyanobacteria. 
Synechococcus is of picoplankton size (refer to Figure 1.2); it occurs 
abundantly in the euphotic zone of both Coastal and oceanic waters of 
températe and tropical oceans. Concentrations of Synechococcus may reach 
up to 10 6 cells mi -1 and, at such high concentrations and in the absence of 
larger phytoplankton, this single genus can play a major role in the primary 
productivity of the sea. Recently scientists have discovered even smaller 
(0.6-0.8 ¡x m diameter) photosynthetic organisms called prochlorophytes, 
which are closely related to the cyanobacteria and occur in both Coastal and 
oceanic waters. Although few ecological studies have been made of these 
organisms, the genus Prochlorococcus apparently contributes to a significant 
fraction of the total primary production in the oceanic equatorial Pacific. 

OH ESTIO N 3,1 Assuming a spherical shape, how many Synechococcus cells of 
1 ftm diameter are equívalent ín volume to a single dinofiagellate ce 11 of 50 
gm diameter? 


Phytoplankton are the dominant primary producers of the pelagic realm 
converting inorganic materials (e.g. nitrate, phosphate) into new organic 
compounds (e.g. lipids, proteins) by the process of photosynthesis and 
thereby starting the marine food chain. The amount of plant tissue build up 
by photosynthesis over time is generally referred to as primary production, 
so called because photosynthetic production is the basis of most of marine 
production. As we will see later in Sections 5.5 and 8.9, there are other 
types of primary production that are carried out by bacteria capable of 
building organic materials through chemosynthetic mechanisms, but these 
are of minor importance in the oceans as a whole. 

Although a number of steps are involved, the Chemical reactions for 
photosynthesis can be very generally summarized as: 


(requiring sunlight) 

6C0 2 + 6H 2 0 - C 6 H l2 0 6 + 60 2 

carbón dioxide water carbohydrate oxygen 


(requiring metabolic energy) 


Carbón dioxide utilized by the algae can be free dissolved CO 2 , or CO 2 
bound as bicarbonate or carbonate ions (see also Section 5.5.2). The total 
carbón dioxide (all three forms) is about 90 mg CO 2 1 _1 in oceanic waters, 
and this concentration is sufficiently high so that it does not limit the amount 
of photosynthesis by phytoplankton. This type of production, involving a 
reduction of carbón dioxide to produce high-energy organic substances, is 
also called autotrophic production; autotrophic organisms do not require 
organic materials as an energy source. Note that this process not only results 
in the production of plant carbohydrate, but it also produces free oxygen 
(which is derived from the water molecule, not from the carbón dioxide). 
The reverse process is respiration, in which there is an oxidative reaction 
that breaks the high-energy bonds of the carbohydrates and thus releases 
energy needed for metabolism. All organisms, including plants, carry out 
respiration. Whereas photosynthesis can proceed only during periods of 
daylight, respiration is carried out during both light and dark periods. 

wavelength (nm) 

Figure 3.4(a) The absorption spectrum of 
chlorophyll a . 

(b) The absorption spectra of the accessory 
pigments fucoxanthin (a xanthophyll) and 
phycocyanin and phycoerythrin (phycobilins). 

Solar energy is used to drive the process of photosynthesis, and the 
conversión of radiant energy to Chemical energy depends upon special 
photosynthetic pigments that are usually contained in chloroplasts of the 
algae. The dominant pigment is chlorophyll a , but chlorophylls b , c, and d 
plus accessory pigments (carotenes, xanthophylls, and phycobilins) are also 
present in many species and some of these pigments can also be involved in 
this conversión. All of these photosynthetically active pigments absorb light 
of wavelengths within the range of about 400-700 nm (PAR), but each 
shows a different absorption spectrum. Figure 3.4a gives the absorption 
spectrum of chlorophyll a, the most commonly occurring pigment; máximum 
absorption takes place in the red (650-700 nm) and blue-violet (450 nm) 
range. Figure 3.4b shows the absorption spectra of several accessory 
pigments. It is often these accessory pigments that dominate over the green 
colour of chlorophyll, and therefore many phytoplankton appear to be 
brown, golden, or even red in colour. 

QUESTiQN 3.2 Some planktonic (and benthic) algae contain large amounts of 
accessory pigments as well as chlorophyll. Refer to Figures 3.4a, b and 
Figure 2.4 and suggest how these pigments may be ecologically importan! 
for the algae concerned. 

When chlorophyll or other photosynthetically active pigments absorb light, 
the electrons in the pigments molecule acquire a higher energy level. This 
energy in the electrons is then transferred in a series of reactions in which 
ADP (adenosine diphosphate) is changed to higher energy ATP (adenosine 
triphosphate), and a compound called nicotinamide adenine dinucleotide 
phosphate (or NADPH 2 ) is formed. These reactions, which are entirely 
dependent on light energy and involve the conversión of radiant energy to 
Chemical energy, are called the light reactions of photosynthesis. 

The light reactions are inextricably linked with a series of reactions that do 
not require light and which are referred to as the dark reactions of 
photosynthesis. They involve the reduction of CO 2 by NADPH 2 and require 
the Chemical energy of ATP to produce the end producís of high-energy 
carbohydrates (usually polysaccharides) and other organic compounds such 
as lipids. Additionally, the reduction of nitrate (N0 3 ") yields amino acids 
and proteins. 

Note that in the reactions of photosynthesis, compounds are formed that 
contain nitrogen and phosphorus as well as the elements supplied by carbón 

dioxide and water. As with all plants, phytoplankton have absolute mínimum 
requirements for these elements. Nitrogen is usually taken up by the 
phytoplankton cell in the form of dissolved nitrate, nitrite or ammonia; 
phosphorus is normally taken up in dissolved inorganic form (orthophosphate 
ions), or sometimes as dissolved organic phosphorus. Other elements may be 
required as well. Dissolved Silicon, for example, is an absolute requirement 
for diatoms in producing the frustule. In addition, vitamins and certain trace 
elements may also be required, with types and amounts depending upon the 
species of phytoplankton. When photosynthetic species require vitamins or 
other organic growth factors, the production is termed auxotrophic. In 
seawater, all of the compounds referred to here are present in relatively low 
concentrations that vary according to the rates of photosynthesis and 
respiration and other biological activities, such as excretion by animáis or 
bacterial decomposition. Therefore the concentrations of these essential 
elements or substances may at times become so low as to limit the amount 
of primary production. These considerations are discussed in Section 3.4. 


Standing stock refers to the number of organisms per unit area or per unit 
volume of water at the moment of sampling. For phytoplankton, this can be 
measured by microscopic cell counts of preserved phytoplankton filtered 
from seawater samples, and the standing stock is given in number of cells 
per volume of water. However, because phytoplankton vary greatly in size, 
total numbers are not as ecologically meaningful as estimates of their 
biomass. Biomass is defined as the total weight (total numbers x average 
weight) of all organisms in a given area or volume. It is possible to count 
numbers and measure volumes of phytoplankton electronically, and this 
method attempts to provide an estímate of phytoplankton biomass, although 
cell volume may not always accurately reflect cell weight. Biomass is then 
expressed as the total volume (total numbers x volumes — mm 3 ) of 
phytoplankton cells per unit volume of water. The distinction between 
standing stock and biomass is not always made evident, however, and often 
the terms are used synonymously. 

Another laboratory method that attempts to estímate phytoplankton biomass 
determines the quantity of chlorophyll a in seawater. This method is often 
used because chlorophyll a is universally present in all species of 
phytoplankton, can be easily measured, and its relative abundance enables 
estimates to be made of the productive capacity of the phytoplankton 
community. A known volume of water is filtered, and plant pigments are 
extracted in acetone from the organisms retained on the filter. The 
concentration of chlorophyll a is then estimated by placing the sample in a 
fluorometer to measure fluorescence, or in a spectrophotometer which 
measures the extinction of different wavelengths in a beam of light shining 
through the sample. The biomass is expressed as the amount of chlorophyll 
a per volume of water, or as the amount contained in the water column 
under a square metre of water surface. 

However, the rate at which plant material is produced, or the primary 
productivity, is of more ecological interest than instantaneous measures of 
standing stock or biomass. The most popular method of measuring 
productivity in the sea is the 14 C method. In this method, a small measured 
amount of radioactive bicarbonate (HCO 3 - ) is added to two bottles of 
seawater containing phytoplankton. One bottle is exposed to light and 


permits photosynthesis and respiration; the other is shielded from all light so 
that only respiration takes place. The amount of radioactive carbón taken up 
per unit time is later measured on the phytoplankton when they are filtered 
out of the original samples. This radioactivity is measured using a 
scintillation counter, and primary productivity (in mg C m -3 h -1 ) is 
calculated from: 

rate of production 

(Rl — Rd) x W 
R x t 


where R is the total radioactivity added to a sample, t is the number of hours 
of incubation, R L is the radioactive count in the ‘light’ bottle sample, and R D 
is the count of the ‘dark’ sample. W is the total weight of all forms of 
carbón dioxide in the sample (in mg C rrT 3 ), and this is determined 
independently, either by titration or from assuming a specific carbón dioxide 
contení related to the salinity of the sample. The productivity is expressed as 
the amount (in mg) of carbón fixed in new organic material per volume of 
water (m -3 ) per unit time (h -1 ); it varies between zero and as much as about 
80 mg C m -3 h -1 . This method is applied to water samples taken from a 
series of depths. In order to calcúlate production throughout the euphotic 
zone and to facilitate comparisons, the results obtained at different depths 
may be integrated to give production in terms of the amount of carbón fixed 
in the water column under a square metre of surface per day 
(g C m -2 day -1 ). If the amount of carbón fixed per unit time is coupled with 
chlorophyll a measurements of biomass, one obtains a measure of growth 
rate in units of time (mg C per mg chlorophyll a per hour); this measure of 
productivity is sometimes called the assimilation índex (see Table 3.2). 

The carbón-14 method described above can be made very precise by careful 
experimental techniques but, at the same time, there is reason to question its 
accuracy. For example, the uptake in the dark bottle (Rd) is assumed to 
represent a blank with which to correct the uptake in the light bottle (Rl). 
This assumes that, except for photosynthesis, the same biological activities 
go on in both the light and dark bottles; but this may not be quite true. Also, 
any soluble organic material that is lost by the phytoplankton (a process 
known as exudation) during the period of photosynthesis will not be 
measured as it is not retained during filtration. Therefore, although the ,4 C 
method is the most practical measurement of photosynthesis in the sea, it 
may sometimes lead to errors. 

Other techniques have been developed for measuring chlorophyll 
concentration and thus relative phytoplankton abundance over large expanses 
of sea. A fluorometer that produces a certain wavelength of ultraviolet light 
will cause chlorophyll to emit a red fiuorescence, and this device can then 
estimate the amount of chlorophyll in a volume of water. The method is very 
sensitive, and a fluorometer towed from a research vessel (see Figure 4.2) 
can rapidly record changes in chlorophyll concentration over large distances 
of sea surface. Remóte sensing by aircraft or satellites provides even broader 
spatial coverage of phytoplankton abundance. This technique is based on the 
fact that the radiance reflected from the sea surface in the visible (or PAR) 
spectrum (400-700 nm) is related to the concentration of chlorophyll. 
Because chlorophyll is green, and water colour changes from blue to green 
as chlorophyll concentration increases, the relative colour diffcrences can be 
used as a measure of chlorophyll concentration (see Colour Píate 8). Satellite 
measurements are not as sensitive as others and have restrictions of limited 


depth penetration, but they provide useful pattems of relative plant 
production on a global scale. 


The amount of light (or solar radiation) strongly affects both the amount and 
rate of photosynthesis. Thus the photosynthesis occurring in a water sample 
is proportional to the light intensity, as shown in Figure 3.5 where 
photosynthesis increases with increasing light intensity up to some maximal 
valué (P max ). At still higher light intensities, there may be a significant 
decrease in photosynthesis (called photoinhibition) that is caused by a 
number of physiological reactions such as shrinkage of chloroplasts in bright 

The point on the curve in Figure 3.5 at which the amount of respiration 
exactly balances the amount of photosynthesis is called the compensation 
point, and this occurs at a compensation light intensity (/ c ) which was 
defined earlier (Section 2.1.2) as marking the lower boundary of the euphotic 
zone. The term gross primary productivity (P g ) is used to describe the 
total photosynthesis, and net primary productivity ( P n ) denotes gross 
photosynthesis minus plant respiration. 

The curve in Figure 3.5 can be described by mathematical equations that 
closely approximate two independent series of reactions, one series (shown 
by the initial slope AP/AI) being the light-dependent reactions of 
photosynthesis and the other (P max ) being the dark reactions, both of which 
were defined in Section 3.2. The simplest equations which describe the curve 
up to P max (i.e. with no photoinhibition) are: 


Ki + [/I 


Figure 3.5 The response of photosynthesis (P) 
to changes in light intensity (I). l c , 
compensation light intensity; K¡, the 
half-saturation constant, or the light intensity 
when photosynthesis equals 1/2 of maximal 
photosynthesis (P max ); P g , gross photosynthesis; 
and P n , net photosynthesis. Absolute units not 
shown because all units are species specific. 

(cal cnr 2 min 11 ) 



PmaxU ~ I C ] 
*/ + [/- Ic\ 


where P¿ and P n are gross and net productivity, respectively, as defined 
above; and K¡ ís the half-saturation constant, or the light intensity when 
P = P max /2. Af/ valúes range from about 10 to 50 jx E mr 2 s' 1 . [I] is the 
amount of ambient PAR light, and f 1 — le] is the ambient PAR light less the 
compensation light intensity, /c- 

In the above equations, it is implied that there is a defined light response for 
all algae growing under constant physiological conditions and that this 
response can be described by two constants, P max and K¡. In fací, different 
species have different valúes of P max and K¡ and, even within the same 
species, the photosynthetic response of a cell to light can change over time 
(e.g. over the course of a day from bright light near the surface to shade 
adaptation deeper in the water column). In general, the initial slope of the 
curve (AP/A/) in Figure 3.5 will respond to physiological changes in the 
photosynthetic biochemistry of a cell (i.e. the light-dependent reactions). The 
upper limit of the curve (P max ) will respond to changes in environmental 
parameters, such as nutrient concentration and temperature, which affect the 
dark reactions of photosynthesis. Because different species of phytoplankton 
respond differently to changes in surface radiation and in sita light intensity, 
changing environmental conditions will favour different species at different 
times and lead to a succession of different dominant species in the 
community. Valúes for P max and AP/A/ are given in Table 3.2. Note that 
Pmax is generally increased at higher temperatures and under high nutrient 
conditions, but the initial slope of the photosynthetic curve (AP/A/) is more 
dependent on cellular properties; e.g. picoplankton generally have higher 
AP/A/ valúes than larger phytoplankton. Consequently, picoplankton can 
grow deeper in the water column where there is less light. 

Table 3.2 Representative valúes of P max and AP/A/. AP/A/ is the initial 
slope of the curve in Figure 3.5 and is given in terms of productivity divided 
by solar radiation; P max is given as the máximum valué of the assimilation 
Índex (see Section 3.2.1). 

Pmax (assimilation index) 

(mg C mg _l Chl a h _l ) 



General range 


Low temperatures, 2-4°C 


High temperatures, 8-18°C 


Low nutrients (e.g. in the 
Kuroshio current) 


High nutrients and high 
temperatures (e.g. in 
tropical Coastal waters) 

AP/A/ (initial slope) 

(mg C mg -1 Chl a h~ l )/(¡xE m -2 s _1 ) 



Températe ocean 


Subtropical waters 


Picoplankton (< 1.0 fx m) 


Annual range for températe 

(annual average, 0.045) 

Coastal waters 


QUESTION 3.3 Using equation 3,2 and assuming a valué of 
2mgC mg l Chl a h 3 and a valué of 10 gE m" 2 s^ 1 for one species 
of phytoplankton and respective P m4X and valúes of 
6 mg C mg _1 Chl a h“ ] and 20 juE s H for a second speeies, which 
species will be growing faster at a PAR light intensity of 50 /xE m : s^ 1 ? 

In an earlier section ( 2 . 1 . 2 ), we considered how to calcúlate the extinction of 
light in water and the compensation depth of light. Equations 2.1 and 2.2 
can now be extended to deal with the problem of phytoplankton being mixed 
vertically in the water column. When phytoplankton are being mixed up and 
down in the surface layers of the sea, it is useful to know the average 
amount of light (¡a) in the euphotic zone. This is given by the expression: 

l D = ^:(\-e- kD ) (3.4) 

where 7 q is the surface radiation, k is the extinction coefficient, and D is the 
depth over which the light intensity is averaged. 

A useful application of equation 3.4 is to consider how far down a 
population of phytoplankton cells can be mixed until photosynthetic gain is 
balanced by respiratory losses (i.e. where P w = R w , Figure 3.6). This depth 
is called the critical depth (Z) cr ). If equation 3.4 is rearranged and le, the 
compensation light intensity, is substituted for 7 o, we get the following 
expression to calcúlate the critical depth: 

¿>cr - ~-(l - e~ kD «) 



If kD cr >> 0, then equation 3.5 can be simplified to: 

D Qr = (3.6) 


The importance of the critical depth is illustrated in Figure 3.6. This figure 
indicates that if the amount of plant material used up in respiration (in the 
area bounded by ABCD) is matched against the amount gained by 
photosynthesis (area ACE), then diagrammatically one arrives at the same 
depth as calculated in equation 3.6, that is, the critical depth. If 
phytoplankton cells are mixed downward below this depth by intensive 
storm action, there can be no net photosynthesis. However, as long as the 
depth of mixing is above the critical depth, positive net photosynthesis can 
occur. Thus by using a simple formula based on the amount of radiation at 
the surface (7 0 ), the extinction coefficient ( k ), and a known compensation 
light intensity (7c), it is possible to estimate when the spring production of 
phytoplankton can start in températe latitudes. 

QUESTION 3.4 The surface radiation is 500 ¡t E m -2 seo -1 of which 5(Wc is 
PAR. the compensation üght intensity of the phytoplankton is 
10 E m -2 sec -1 , and the depth of mixing in the water column is 100 m. 
Using the extinction coefficient obtained in Question 2.2, is there any net 
positive photosynthesis ín this water column? 


Figure 3.6 An illustration of the relationships among the compensation light depth, the 
critical depth, and the depth of mixing. At the compensation depth (De), the light intensity 
(le) is such that the photosynthesis of a single cell (Pe) is equa) to its respiration (R c )\ 
above this depth there is a net gain from photosynthesis (P c > fíe) and below it there is a 
net loss (Pc < Re)- As phytoplankton celis are mixed above and below the compensation 
depth, they experience an average light intensity (f D ) in the water column. The depth at 
which l D equals le is the critical depth (D cr ) where photosynthesis throughout the water 
column (P w ) equals phytoplankton respiration throughout the water column (R w ). The area 
bounded by points A, B, C and D represents phytoplankton respiration, and the area 
bounded by points A, C and E represents photosynthesis; these two areas are equal at the 
critical depth. When the critical depth is less than the depth of mixing (Dm) (as illustrated in 
this figure), no net production takes place because P w < fí w . Net production of the 
phytoplankton (P w > R w ) only occurs when the critical depth lies below the depth of mixing. 


In Section 3.2.1, productivity was represented as the amount of carbón fixed 
per unit time. This is a convenient convention because it is what the 
ecologist actually measures. It was also pointed out that productivity can be 
represented by the assimilation Índex, in which growth is expressed as mg of 
carbón produced per mg of chlorophyll a per hour. This valué is useful for 


comparing photosynthesis from different areas because it normalizes all 
measurements to a unit of chlorophyll a . 

QUESTIONi 3.5 In comps iring two difieren! areas of the ocean, we tind (hal the 
phoiosynthetic production is 20 mg C ni 3 h ~ l in area A and 
50 mg Cm‘ 3 h 1 ín area B, The standing stocks of phytoplankton are 
2 mg Chl a m 1 and 25 mg Chl a m 3 in areas A and B, respectively. 

(a) In which area are !he phytoplankton most photosynthetícally active? (Use 
assimilation indices to determine the answer.) (h) What could cause this 
difieren ce in aclivity? 

Another useful way of comparing growth rates of phytoplankton is to 
express growth as an increase in cell numbers. For unicellular organisms, 
this is an exponential function: 

(X 0 + AX) = X 0 ^ (3.7) 

where X 0 is the population of cells at the beginning of the experiment, AX 
is the number produced during time í, and /x is the growth constant of the 
population per unit of time. If AX has been measured in units of 
photosynthetic carbón, then Xq musí be expressed as the total standing stock 
of phytoplankton carbón instead of in terms of cell numbers. 

One additional expression that can be obtained from equation 3.7 is the 
doubling time, which is defined as the time taken for a population to 
increase by 100%. Doubling times for phytoplankton can be derived from: 

X, = X 0 e* lt (3.8) 

where X, is (X 0 + AX) in equation 3.7. The time required for Xo to double 
(d) is given as, 

and then doubling time (d) can be calculated from: 

i = log e 2 _ 0.69 
{l /x 



The reciprocal of doubling time (or 1 /d when d is in days) gives the 
generation time as number of generations produced per day. 

The effect of nutrient concentration on the growth constant, /x, can be 
described by the same expression that was used for photosynthesis 
(equations 3.2 and 3.3). Henee, 

Mmax [N] 

K n + m 


where ¡i is the growth rate (time -1 ) at a specific nutrient concentration [N] 
which is usually expressed in micromoles (/xM) per litre, /x max is the 
máximum growth rate of the phytoplankton, and K N (given in /xM) is a 
half-saturation constant for nutrient uptake that is equal to the concentration 
of nutrients at 1/2 /x ma x- 

Equation 3.11 is valid when the growth rate of phytoplankton is controlled 
by the nutrient concentration in seawater. However, in some surface waters 





Figure 37 Three possible variations (a, b, and 
c) in the nutrient—growth curves of competing 
pairs (species 1 and 2) of hypotheticaí species 
of phytoplankton. ¡s the specific growth rate; 
/¿ max is the máximum rate of growth; K N is a 
half-saturation constant for nutrient uptake; [A/| 
is ambient nutrient concentraron; SI is species 
1; and S2 is species 2. All units are arbitrary. 
See text for discussion of the differences in 
resuits between competing species. 

with extremely low concentrations of nutrients, some larger photosynthetic 
dinoflagellates (See Section 3.1.2) can migrate to deeper layers where 
nutrients are more abundant. The zone where nutrient concentrations 
increase rapidly with depth is the nutricline, and this may be below the 
euphotic zone. After taking nutrients such as nitrate into the cell, these 
flagellates can return to sunlit waters to carry out photosynthesis. In such 
cases, the (photosynthetic) growth rate of the phytoplankton is proportional 
to the nutrients within the cell, and not to the external nutrient concentration. 

Among the principal nutrients in the sea that are required for phytoplankton 
growth, only certain elements may be in short supply. In general, the 
quantities of magnesium, calcium, potassium, sodium, sulphate, chloride, etc. 
(Table 2.1) are all in sufficient quantities for plant growth. Carbón dioxide, 
which may be limiting in lake waters, is present in excessive quantities in 
seawater. However, some essential inorganic substances, like nitrate, 
phosphate, silicate, iron, and manganese, may be present in concentrations 
that are low enough to be limiting to plant production. There may also be 
synergistic effects between essential nutrients. For example, the 
concentration of iron in a metabolizable form governs the ability of 
phytoplankton to utilize inorganic nitrogen. This is because iron is required 
in the enzymes nitrite reducíase and nitrate reducíase, and these enzymes are 
necessary for the reduction of nitrite and nitrate to ammonium, which is used 
to make amino acids. Large diatoms may be affected by iron limitation, but 
small flagellates usually are not because they can take up iron at lower 
concentrations. Ocean areas that are limited by iron are characterized by 
having high nitrate but low chlorophyll concentrations, and they are referred 
to as HNLC areas. They inelude the subarctic North Pacific, Equatorial 
Pacific, and parts of the Antarctic Ocean. In addition, certain organic 
substances (e.g. vitamin B 12 , thiamine, and biotin) are required for 
auxotrophic growth of some phytoplankton, and these substances may also 
be in short supply in seawater and thus limiting to growth. 

Many different species of phytoplankton can be found living in the same 
water mass. Whut fació rs al low the coexistence of so many species, all of 
which have the same hasje requi reme rus for sunlight carbón dioxide* and 
nutrients. and all of which may compete for requi re me nts that are ¡n limited 

Each phytoplankton species has a specific half-saturation concentration {K N 
in equation 3.11) for the uptake of each of the limiting nutrients, and each 
species has a different máximum growth rate (/¿ max ). These species-specific 
differences in growth rates and responses to nutrients allow a great variety of 
phytoplankton to grow in what seems to be a very uniform environment. 

This is illustrated in part by Figure 3.7 which shows changes in growth rates 
(/x) of different hypotheticaí species of phytoplankton having different valúes 
of K n and ¿t max and responding to variations in the ambient concentration of 
one nutrient (see equation 3.11). In the first example (a) of this figure, 
species 1 has a higher máximum growth rate than species 2. Because Kn is 
the same for both species, they grow at the same rate to a certain level of 
nutrient, beyond which species 1 continúes to its higher máximum growth 
rate. In a second example (b), two different species have the same valué for 
¡i max , but they achieve this at different nutrient concentrations. The valué of 
Kn is lower for species 1, so it reaches the máximum growth rate at a lower 
nutrient concentration. In the last example (c), two other competing species 
have differing valúes for both ¿x max and K N , and the competitive advantage 


Figure 3.8 Coexistence of phytoplankton 
species when limited by two resources. (a) One 
species (1) which ¡s limited only by the two 
lower concentrations of each resource. (b) Two 
species (1 and 2) showing the región of 
coexistence on each rate-limiting resource. 

(c) Four species showing regions of coexistence 
based on each resource: 1 is the best 
competitor for resource 1; 2 is the second best; 
3, third; and 4, fourth; the competitive rank 
order for resource ii is reversed. A circled 
number indicates that only that species can 
exist under the given circumstances. 

shifts between the species as the nutrient concentration changes. At lower 
nutrient concentrations, species 2 dominates because it grows faster; but at 
higher nutrient concentrations, species 1 becomes dominant because it 
achieves a higher máximum rate of growth. 

If one considers further that two, three, or many growth-rate limiting 
nutrients may occur in any body of water, and that there are also differences 
in light and other physical properties such as temperature and salinity, it is 
obvious that there is a constantly changing mosaic of rate-limiting factors 
governing the growth of phytoplankton. Since each species responds 
differently to the mosaic, and since growth cannot be limited by more than 
one process at any one time, the physical/chemical restrictions on 
phytoplankton growth allow for the coexistence of many species in the same 
body of water, with successive changes in the relative abundance of the 
component species. 

Figure 3.8 further explores how several nutrients and several species can 
interact to produce the diverse phytoplankton populations discussed above. 

In Figure 3.8a, a single species (1) of phytoplankton is considered in relation 
to two potentially limiting resources (e.g. nutrients such as nitrate and 
phosphate). The species requires a certain minimum concentration of each 
nutrient ( R \* and Rn* valúes). If the concentrations drop below these levels, 
species 1 cannot exist even without competition from another species; above 
these minimal nutrient levels, species 1 can survive and grow. If a second 




without competition: 
species 1 persists 


without competition: 

1 species leliminated 



' resource 1 mareases —► 




phytoplankton species (2) having different nutrient-concentration requirements 
is introduced (Figure 3.8b), the situation becomes more complex. In 
this hypothetical example, each species can be limited by a different nutrient 
concentration. Species 1 is a superior competitor for resource 1; it will 
be the only species capable of existing at very low levels of resource 1, and 
it will outcompete and exelude species 2 at slightly higher concentrations 
of this resource. Conversely, species 2 is the superior member and only 
survivor when resource 2 is in low concentrations. Above these minimal 
nutrient levels, there is a región where both species can coexist. As more 
species, each with its own nutrient requirements, are added to the community 
(Figure 3.8c), there are more possibilities of establishing coexistence. 

With reference to the examples in Figure 3.8, if both nutrients are abundant 
and phytoplankton growth is not limited by any of the K¡^¡ valúes for nutrient 
uptake, then species dominance will be determined by the /x max valúes of the 
species and the fastest growing phytoplankton species will dominate. In the 
extreme case of very high nutrient concentrations, there will be a single 
species, the one with the highest ax . Thus at both extremes of the nutrient 
field, very low and very high nutrient levels tend to lead to a low diversity 
of phytoplankton species. Very low nutrient concentrations can lead to 
dominance of the community by a single species with the lowest K N , and 
very high nutrient conditions can lead to dominance by a single species 
having the highest /x max . If Ihe simple illustrations in Figure 3.8 are 
expanded to inelude additional resources and other species, an almost infinite 
combination of physical/chemical backgrounds is produced in which many 
phytoplankton species can grow. 

The physico-chemical environmental mosaic itself is not constant. The light 
and temperature background changes daily and seasonally, and nutrient 
concentrations vary. Sometimes it is the change itself that affeets different 
phytoplankton responses. For example, nutrient concentrations may change 
sporadically by pulsing inputs resulting from diel upwelling of deeper water 
with high nutrient levels. Such fluctuadng changes in nutrient concentration 
will have a different effect on phytoplankton species composition than when 
nutrients are maintained at relatively constant levels through, for example, 
sustained upwelling. On the other hand, toxic pollutants will work in the 
opposite direction to nutrient resources; at higher concentrations, they will 
selectively inhibit the growth of certain phytoplankton species, so that 
eventually diversity is reduced to only the most pollutant-resistant forms. It 
must also be added that selective grazing by herbivorous zooplankton can 
alter the relative abundance of phytoplankton species. 

Some valúes for growth rates (/x max ) and half-saturation constants (Kn) for 
phytoplankton are given in Table 3.3. The relative availability of nutrients 
for phytoplankton (particularly of nitrate and ammonium which are most 
often present in limiting quantities) can be used to classify aquatic 
environments. Regions that have low concentrations of essential nutrients, 
and therefore low primary productivity, are called oligotrophic. Such areas 
typically have chlorophyll concentrations ranging from <0.05 gg l' 1 at the 
surface to a máximum of 0.1-0.5 gg l -1 at depths of 100-150 m. 

Eutrophic waters contain nutrients in high concentrations; high 
phytoplankton densities are manifested by chlorophyll concentrations of 1 to 
10 gg 1 _I in surface layers. Mesotrophic is a term that is sometimes applied 
to waters of intermediate character. Eutrophic waters tend to be dominated 
by one or two fast-growing, r-selected phytoplankton species (see 
Table 1.1). In contrast, oligotrophic waters tend to have many competing 


Table 3.3 Máximum growth rates (¿i max ) and half-saturation constants (K N ) for 
some phytoplankton. 

Máximum growth rates (¡i max ) 
(in generations day~ l ) 



Oligotrophic, tropical waters 


Températe, eutrophic Coastal 



Tropical upwelling; and 
picoplankton under 
eutrophic conditions 
and high temperatures 

Half-saturation constants ( Kv) 
(in /xM) 

Nitrate or ammonium 


Oligotrophic waters 


Eutrophic oceanic waters 


Eutrophic Coastal waters 



General range for diatoms 



General range for olig¬ 
otrophic to eutrophic waters 

/c-selected species, each dependent on a different limiting nutrient; the 
community thus tends to be in equilibrium with the total nutrient supply. 

QUESTIQN 3.6 From Table 3.3. what is the general relatíonshíp between the 
definítions oí eutrophic and oligotrophic and the half-saturation constants 
{K¡y) of phytoplankton celia? What dees ibis itnply in rerms of relative 
nutrient uptake bv the phytoplankton living in eutrophic or oligotrophic 


Light is one of the two major physical factors controlling phytoplankton 
production in the sea. The second ineludes those physical forces which bring 
nutrients up from deep water, where they accumulate, into the euphotic zone. 
These two features together largely determine what type of phytoplankton 
develop and how much primary production occurs in any part of the world’s 
ocean. They are also major factors in determining the amount and type of 
marine animáis that are produced, including fish which are caught 

The amount of light decreases from the Equator towards the poles. On the 
other hand, the amount of wind mixing, which brings nutrients up to the 
surface, increases from the tropics (where water is vertically stabilized by 
solar heating) toward the poles. Thus the abundance of light and the 
abundance of nutrients in the euphotic zone form an inverse relationship 
(Figure 3.9) which largely determines the pattern of phytoplankton 
production in different latitudes. In polar regions, a single pulse of 
phytoplankton abundance occurs during the summer when light becomes 
sufficient for a net increase in primary productivity. In températe latitudes, 


primary productivity is generally maximal in the spring and again in the 
autumn when the combination of available light and high nutrient 
concentrations allows plankton blooms to occur. In the tropics, where intense 
surface heating produces a permanent thermocline (see Section 2.2.2), the 
phytoplankton are generally nutrient-limited throughout the year, and there 
are only small and irregular fluctuations in primary production due to local 

Figure 3.9 is a general representation of the annual cycle of phytoplankton 
production in the world’s ocean. However, there are many physical features 
that affect nutrient levels in the euphotic zone and thereby greatly modify 
the general pattern. These inelude fronts, which are relatively narrow 
regions characterized by large horizontal gradients in variables such as 
temperature, salinity, and density, and eddy-formations such as rings and 
large-scale gyres, which have characteristic rotational patterns of circulation. 
These modifying physical features may be thousands of kilometres wide 
(e.g. gyres) or only a few kilometres long (e.g. tidal and river-plume fronts). 
The size depends on the topography and ocean climate of any particular 

Figure 3.9 The relative abundance of light (unshaded area) and nutrients (shaded area) at 
the sea surface and the relative seasonal change in primary productivity at three different 
latitudes. (Productivity expressed in arbitrary vertical scales.) 





Températe productivity 

Tropical productivity 








location. The common feature of all these structures is that there is some 
mechanism involved for bringing nutrients up to the euphotic zone from 
deeper water, on time scales which may range from days to months. These 
mechanisms are superimposed on the seasonal wind mixing that partly 
generates the global pattern of phytoplankton production shown 
schematically in Figure 3.9. Some of the nutrient-enhancing processes can 
result in ‘oases’ of plankton production during periods of the year when the 
production of phytoplankton would otherwise be low. 


The general circulation of surface water in the global ocean (discussed in 
Section 2.6 and shown in Figures 2.19 and 3.10) results in large gyres. In 
the anticyclonic gyres, water flows in a clockwise direction in the Northern 
Hemisphere, and in an anticlockwise direction in the Southern Hemisphere 
(see Table 3.4). In the Northern Hemisphere, the clockwise flow results in 
convergent gyres because the direction of water circulation tends to draw 
surface water in toward the centre. This is illustrated in Figure 3.11b where 
it can be seen that anticyclonic gyres in the Northern Hemisphere tend to 
deepen the thermocline due to the convergent tendency of the circulation. In 
this situation, no new nutrients can come to the surface from the deep water, 

Figure 3.10 The location of upwelling zones and coral reefs in the world’s ocean. 


Figure 3.11 Plan and cross-sectional views of 
a cyclonic (a) and aníicyclonic (b) gyre in the 
Northern Hemisphere. The dashed arrows 
indícate net transport of water away from and 
towards the centre, respectively. The same 
pattern of circulation applies to warm core and 
coid core rings, but on a smaller scale. 

Table 3.4 Water flow in gyres and rings in the Northern and Southern Hemispheres. 

Cyclonic gyres 

Anticyclonic gyres 



Coid core rings 

Warm core rings 



leading to 

leading to 

high production 

low production 





and convergent gyres like the Sargasso Sea in the North Atlantic are relative 
‘deserts’ of ocean production. In the Southern Hemisphere, the reverse 
rotational direction of the gyres also reverses the vertical flow of water 
within the system, so that the anticlockwise circulation also forms 
convergent gyres with relatively low productivity. 

Cyclonic gyres are formed by water circulating in an anticlockwise direction 
in the Northern Hemisphere and in a clockwise direction in the Southern 
Hemisphere. These are divergent gyres, which tend to draw water up from 
below the thermocline (Figure 3.11a); this results in a plentiful supply of 
nutrients at the surface that should make such areas highly productive. The 
Alaskan Gyre in the Gulf of Alaska is a divergent gyre in which the actual 
vertical movement of water from below the thermocline is believed to be 
about 10 m yr _I . Although this would be a highly productive gyre if situated 
farther south, its location at north of 50° N means that the area is limited by 
light in winter, and the productivity of the gyre is actually controlled more 
by seasonal events than by oceanic circulation. 

gyre gyre 



no upweliíng 

convergen ce 


direction oí 
current flow 


Figure 3.12 The sequential formation of 
warm (W) and coid (C) core rings from a major 
current system (such as the Gulf Stream) in the 
Northern Hemisphere. As the current flows 
between water of contrasting temperature, it 
begins to develop a meandering pattern (a) with 
eddies forming on the edges. As the meanders 
become more pronounced (b), the eddies are 
eventually pinched off to form independent 
circulatory Systems called rings (c). Note that 
this results in warm water rings being isolated 
in areas of coid water, and in coid water being 
transferred across the current into an area of 
predominantly warm temperatures. 

Ring structure in the ocean have the same morphology as gyres, but they are 
much smaller, being hundreds, rather than thousands, of kilometres in 
diameter. They are formed as eddies that detach from a major current system 
such as the Gulf Stream. Such large currents tend to meander and, in so 
doing, large eddies or rings spin off as independent bodies of circulating 
water which may survive for several years (i.e. long enough to influence the 
primary productivity within the ring). The two types of rings shown in 
Figures 3.12 and Colour Píate 7 are referred to as warm core rings 
(anticyclonic) and coid core rings (cyclonic). A cross-section of each ring 
type would look like the cyclonic and anticyclonic gyres shown in 
Figure 3.11, but on a much smaller scale. The rotational circulation of the 
rings maintains cooler (cyclonic circulation) or warmer (anticyclonic) 
temperatures because of the respective vertical flow of water within the rings 
(see Table 3.4 and Figure 3.11). However, although the isotherms in coid 
core rings bow up in the middle as they do in cyclonic gyres (Figure 3.11), 
this does not necessarily signify upwelling. High productivity within coid 
core rings may result because the water which has been captured by the 
meander is already nutrient-rich. Similarly, water in the centre of warm core 
rings is not necessarily sinking. 


Very large frontal zones occur along the edges of continents due to 
wind-driven oceanic circulation. Major divergent continental fronts are 
associated with the Perú Current and California Current in the Pacific, and 
with the Canaries Current and Benguela Current in the Atlantic 
(Figure 2.19). Currents such as these that flow toward the Equator along the 
western coasts of continents are driven away from the coasts due to the 
EartlTs eastward rotation, and this consequently leads to Coastal upwelling 
(Figure 3.10). Upwelling of nutrient-rich water in these areas continúes for 
many months of the year. Further, the location of these currents in latitudes 
between 10° and 40° means there is generally enough solar radiation to 
allow máximum photosynthesis during most of the year. These four 
divergent continental fronts are among the most productive regions in the 
ocean. They are characterized by having large populations of fish and birds, 
and they have been the subject of much scientific investigation because of 
their exploitable resources. 

Another divergent continental front exhibiting upwelling and extremely high 
production occurs around the continent of Antárctica. Known as the 
Antarctic Divergence, this area is the home of huge stocks of krill and 
other zooplankton which give rise to abundant stocks of whales, seáis, and 
seabirds (see Section 5.2 and Figure 5.4). 

Contrary to expectations, the west coast of Australia does not support a large 
fishery that would be indicative of upwelling. Although water does have a 
tendency to upwell on this western coast, the upwelling is suppressed by a 
continual strong flow of warm water from the north which covers the area 
(Figure 3.10). A similar flow of warm water across the Pacific Ocean can 
sometimes suppress the effect of the Peruvian upwelling by greatly 
increasing the depth of the thermo-nutricline, an event that has become 
known as an El Niño. 

An opposite type of convergent continental front tends to form on the 
eastern sides of continents, where water flows away from the Equator. These 


regions are characterized by the accumulation of large quantities of warm, 
nutrient-poor water. They are usually areas where coral reefs occur in 
máximum abundance (Figure 3.10); these inelude the Great Barrier Reef off 
eastern Australia in the South Pacific, the coral reefs of Madagascar in the 
Indian Ocean, and the reefs of the Caribbean Sea. 


The continental frontal systems described above are large enough to be 
described as planetary fronts, but they have been dealt with separately 
because of their very special association with continents. Other planetary 
frontal systems are formed by the convergence or divergence of two current 
systems which often have contrasting properties. Thus the Oyashio off the 
northern coast of Japan is a coid nutrient-rich current that meets the warm 
and vertically stable Kuroshio in the western Pacific (Figure 2.19). These 
two currents join to form the North Pacific Current which flows from Japan 
to the west coast of North America. Mixing of these waters produces a very 
large frontal zone that is highly productive for marine life. A similar 
situation occurs in the North Atlantic where the coid Labrador Current meets 
the warm Gulf Stream (Figures 2.19 and Colour Píate 7). 

A planetary frontal system is also formed around the Antarctic continent at 
latitudes of about 57°-59°S; here there is a convergence of subtropical water 
with Antarctic water, forming the Antarctic Polar Front (or Antarctic 
Convergence). This convergent zone of sinking water is an important source 
of coid deep water for the world’s ocean (see Section 2.4 and Figure 2.17). 

Finally, the last of the fronts which can affect productivity on a planetary 
scale is the upwelling caused largely by divergent current patterns across the 
Equator. Equatorial upwelling is particularly pronounced at about 10°N in 
the Pacific Ocean, where it results in an extensión of the Californian and 
Peruvian continental upwellings out into the Pacific Ocean. It also occurs in 
the Atlantic and to a lesser extent in the Indian Ocean. 


Shelf-break fronts occur along the edges of continental shelves and other 
banks (which are often undersea ‘island’ extensions of the continental 
shelves). A shelf-break front is formed by a combination of the sudden 
shallowing of water across a continental shelf, and by the change in current 
speed across the shelf which may be induced by residual oceanic circulation 
or, especially, by tidal exchange. The process by which a shelf-break front is 
formed can be analysed by considering the ratio (R) of the potential energy 
(PE) in maintaining well-mixed conditions to the rate of current energy 
dissipation ( TED ) in a water column of unit cross-sectional area: 

R = 




The two forms of energy (PE and TED) can be formulated in terms of a 
number of parameters, most of which are constant for a defined area where 
the major form of stratification is a thermocline. Two important terms which 
are not constant are the average water velocity, \U\, and water depth, h. 
These are considered in formulating a stratification Índex expressed as: 


-=-(in c.g.s. umts) 

Cz>|t /| 3 

5 = logio 



Figure 3.13 (a) Average stratification índex 
valúes obtained for the Celtic Sea between 
Ireland and England. 

(b) Surface distribution of chlorophyll a, in 
April, for the Celtic Sea. 

where C& is a frictional or drag coefficient that can be approximated as a 
constant ( ca . 0.003) for a sandy bottom. The stratification Índex can be 
easily calculated for any Coastal región, and it usually falls within the range 
of + 3and — 2, the former valué indicating highly stratified water and the 
latter, highly turbulent, A valué of S ^ 1.5 provides the best conditions for 
phytoplankton production, indicating water that is not too stratified and not 
too turbulent. Nutrients that are brought to the surface by turbulence as the 
water velocity increases over a shelf or bank can be utilized by the 
phytoplankton, resulting in a shelf-break front of high primary productivity. 
Note, however, that the highest standing stock of phytoplankton (i.e. 


dislance (nautical miles) 

Figure 3.14 (a) Chlorophyll a valúes through a frontal región. 

(b) Corresponding temperature (°C) through a frontal región. 

chlorophyll a concentration) will develop over time on the more stable side 
of the front. 

Shelf-break fronts are illustrated in Figures 3.13 and 3.14. Calculations of 
the stratification Índex (Figure 3.13a) coincide spatially with a chlorophyll 
máximum (Figure 3.13b) in the Celtic Sea. In Figure 3.14, the vertical 
distribution of chlorophyll is shown within a mixed water column on one 
side of the front and within a stable water column on the other side of the 

CUESTION 3.7 The mean tidal flow across a shallow bank havíng a mínimum 
depth of 50 m is 3.3 cm s _l T Assuming a sandy bollom. will ibis bank 
produce a frontal /one? 


Rivers entering the sea often carry high nutrients, derived either from natural 
sources or from agricultural fertilizers and sewage. These nutrients enrich 



Figure 3.15 Nutrient enlrainment at the mouth 
of a river (a) Cross-sectional view. (b) Plan 

River plume 

brackish water 

Coastal waters and increase productivity off the mouth of the river. In 
addition, estuarine waters are often highly productive because the flow of the 
river at the sea surface causes nutrients to be entrained from deeper water 
(Figure 3.15) upwelling into the surface water. Providing the deep waters are 
rich in phosphates and nitrates, the entrainment of nutrients also contributes 
to phytoplankton blooms off the river mouth. The exact position of a bloom 
in the river plume (or the location of the front) is a function of many factors 
including the quantity of nutrients introduced and/or entrained, the settling 
out of river silt which allows light to penétrate deeper, the depth of the 
mixed layer, grazing by zooplankton, etc. A phytoplankton bloom may also 
be disrupted or enhanced by the prevailing oceanic climate affecting the 


In addition to the five major physical processes that bring nutrients up to the 
euphotic zone as discussed above (Sections 3.5.1-3.5.5), there are many 
additional minor effects that form smaller frontal zones by physically 
altering nutrient concentrations in surface waters. Among these is the island 
mass effect (also known as island wake effect). This is a disturbance in the 
flow of water caused by the presence of an island (or an undersea mountain), 
resulting in upwelling from below the thermocline and subsequent nutrient 
enrichment of surface waters. First described from enhanced phytoplankton 
biomass and production around Hawaii, it is now known in many localities. 
For example, a plume of high production (> 4 mg Chl a m~ 3 ) extends west 
of the Scilly Isles (off south-west England) for about 50 km into water that 
otherwise contains less than 0.5 mg Chl a m” 3 . Similar upwelling and 
enhanced production can result as currents pass headlands and bays on a 
rugged coastline. 

A different process affecting production on a smaller scale is Langmuir 
circulation. This pattern of circulation is set up when wind blows steadily 
across the surface of relatively calm seas. As a result, vórtices of several 


Figure 3.16 Langmuir vórtices and plankton 
distributions. Neutrally buoyant partióles are 
randomly distributed, but downward swimming 
organisms are aggregated in high velocity 
upwellings (A); partióles that tend to float are 
aggregated in downwellings (B); partióles that 
tend to sink are aggregated in upwellings (C); 
upward swimming organisms are aggregated in 
low-velocity downwellings (D); and horizontally 
swimming organisms are aggregated where 
there is less relative current velocity than within 
the vórtices (E). 

metres in diameter start to revolve around horizontal axes and lead to both 
upwelling and downwelling of water (Figure 3.16). The vertical scale of this 
interaction is not large enough to bring nutrients up from deep water, but it 
is sufficient to concéntrate plankton and this will enhance feeding 
interactions and result in a faster regeneration of nutrients. The phenomenon 
of Langmuir circulation is often visibly apparent as a series of parallel foam 
lines extending for great distances. In the Sargasso Sea, the seaweed 
Sargassum lines up in windrows in response to this type of circulation. 


The primary productivity of phytoplankton in various areas of the global 
ocean varíes with season and location. The highest productivity valúes of 
>1 g C nrT 2 day -1 are encountered in upwelling areas (see Section 3.5.2), 
and the lowest valúes (<0.1 g C m -2 day -1 ) occur in the subtropical 
convergent gyres (see Section 3.5.1). During the summer in subarctic 
latitudes of the Pacific and Atlantic oceans, daily primary productivity may 
be >0.5 g C m -2 , but during the winter there may be no net primary 
productivity for several months. Integration of these different valúes on an 
annual basis gives the range of primary productivity valúes shown in 
Table 3.5. These differences in relative production can also be seen in 
Colour Píate 8 which shows relative chlorophyll concentrations in surface 
waters of the global ocean as detected by remóte sensing from satellites. In 
total, the primary productivity of the world’s ocean is about 40 x 10 9 tonnes 
of carbón per year. This figure is the same order of magnitude as for 
photosynthetic production by terrestrial plants, but the pattern of production 
is very different. 

QUESTIQN 3.8 Much of the ludían Ocean between latitudes 0 and 40 S has 
a low primary productivity of less than 150 mg C m -2 day -1 . What 
feaíure(s) limit production in this ocean? 

In the terrestrial ecosystem, very high productivities occur in relatively small 
areas and ■ -e range of valúes for production are very great. For example, the 


Table 3.5 The range of annual primary productivity in different regions of the 
global ocean. 


Mean annual primary productivity 
(g C m -2 year -1 ) 

Continental upwelling 


(e.g. Perú Current, 

Benguela Current) 

Continental shelf-breaks 


(e.g. European shelf, Grand 

Banks, Patagonia shelf) 

Subarctic oceans 


(e.g. North Atlantic, 

North Pacific) 

Anticyclonic gyres 


(e.g. Sargasso Sea, 
subtropical Pacific) 

Arctic Ocean (ice-covered) 


estimated primary productivity of a rainforest is 3500 g C m -2 year -1 , or 
about six times the highest phytoplankton productivity. On the other hand, 
much of the terrestrial land mass is desert with little or no photosynthetic 
production. In contrast, marine productivity occurs virtually everywhere in 
the euphotic zone of the oceans (covering > 70% of the planet’s surface), 
even under polar ice. It is the accumulative effect of the marine primary 
productivity throughout the world ocean that adds up to a total annual 
production of photosynthetic carbón approximately equivalent to that on 

Latitudinal and seasonal differences in marine productivity result from 
differences in light and nutrient availability (see Figure 3.9). These physical 
forces largely determine the maximal phytoplankton production that is 
possible in any marine area. There are also biological processes that modify 
regional primary production levels. As algae grow, they reduce nutrient 
concentrations in the euphotic zone, and their own increasing numbers create 
self-shading by reducing the penetration of light, thus causing the euphotic 
zone to become shallower. Balancing these effects are the grazing activities 
of herbivorous zooplankton which remove part of the production, and there 
are regional differences in how the phytoplankton community is utilized by 
these animáis. 

When primary productivity increases, it is generally accompanied by a 
measurable increase in the standing stock of phytoplankton. During a bloom 
in coastal areas, the standing stock of chlorophyll a may increase from less 
than 1 mg m -3 to more than 20 mg m -3 over a period of several days. In 
some areas, however, the zooplankton may graze the phytoplankton as fast 
as it is produced, with the result that the increase in primary productivity 
does not show any discernible increase in the standing stock of 
phytoplankton. This situation is found in the North Pacific Ocean at about 
50° N (Figure 3.17). Here, outside of coastal influences, there is virtually no 
change in the standing stock of phytoplankton throughout the year; it remains 
constant at about 0.5 mg chlorophyll a m -3 . However, primary productivity 
in this area increases from winter valúes of less than 50 mg C m -2 day' 1 
to more than 250 mg C m ' 2 day -1 in July. The excess primary productivity 
is grazed by the indigenous zooplankton which increase their biomass as 



North Pacific ^ Tropical 


Figure 3.17 Summary of annual cycles in plankton communities in different regions. The 
solid black lines represent changes in phytoplankton biomass; the dashed blue lines indícate 
changes in zooplankton biomass (arbitrary units). 

indicated in Figure 3.17. The cióse phasing between phytoplankton and 
zooplankton also has implications for deep-sea benthos in the North Pacific 
as there is little uneaten phytoplankton sinking into deep water to serve as a 
food supply for benthic animáis (see Section 8.8.4). 

In contrast, in the Altantic Ocean at the same latitude, the spring bloom is 
characterized by a ten-fold increase in chlorophyll a from 0.1 to about 
1.0 mg m -3 . Primary productivity increases as in the Pacific Ocean, but the 
zooplankton are less efficient at keeping pace with increases in primary 
production. Because only a fraction of the phytoplankton are eaten, there is 
an increase in the standing stock as measured by chlorophyll a. In much of 
the North Atlantic there is also an autumn bloom of phytoplankton, shown in 
Figure 3.17 as a second peak in phytoplankton and zooplankton biomass. As 
much of the phytoplankton is not eaten in North Atlantic waters, decaying 
blooms sink into deep water and become a food source for animáis on the 

Two other annual cycles of phytoplankton and zooplankton are shown in 
Figure 3.17. One shows the pattern in the Arctic Ocean, where a single pulse 
of phytoplankton occurs soon after the disappearance of the ice and is 
followed somewhat slowly by a single pulse in the biomass of zooplankton. 
The lag in response time of the zooplankton to increased food is due to 
relatively slow growth rates in coid water. In tropical environments, the 
biomass of phytoplankton and zooplankton shows no substantial change 
throughout the year. However, storm activities can disrupt this otherwise 
very stable environment so that small pulses in plankton biomass may occur 
irregularly throughout the year. In warm tropical water, any increase in 
phytoplankton standing stock is quickly tracked by the fast-growing 

Primary productivity varíes with depth, and the vertical distribution of 
phytoplankton may change seasonally. This is illustrated over a time 


Figure 3.18 Schematic seasonal depth 
changes in phytoplankton biomass (S), daily net 
photosynthetic rate (P n ), and nutrient 
concentration (N) in stratified températe water. 

S (shaded area), usually expressed in mg chl a 
rcr 3 , P n (broken line), usually expressed as mg 
C per mg Chl a per day, N (blue line), usually 
expressed as (i M nitrate. The figure omits any 
changes caused by significant zooplankton 

sequence in Figure 3.18. In températe latitudes, phytoplankton will be well 
mixed in the surface layer during the winter months and any photosynthesis 
will follow a light attenuation curve (Figure 2.5) except for some 
photoinhibition near the surface. As the spring progresses, primary 
productivity will increase near the surface, and this may be accompanied by 
an increase in the standing stock of phytoplankton. In late summer as the 
System runs out of nutrients near the surface, máximum primary productivity 
will shift deeper in the water column, resulting in a chlorophyll máximum at 

In stable water masses (i.e. most tropical and subtropical oceans), the 
vertical distribution of nutrients, primary productivity, and chlorophyll a 
resembles that shown for late summer in Figure 3.18 and is characteristic of 
the water mass throughout the year. Chlorophyll maxima in such waters can 
be found anywhere from 20 m to over 100 m depending on the long-term 
stability of the water. Under these conditions, the euphotic zone is really 
divided vertically into two communities. The top community is 
nutrient-limited and largely govemed by biological and Chemical processes 
that regenérate nutrients within the zone. The bottom community is 
light-limited, but it is located at the nutricline, where the máximum change 
in nutrient concentration occurs, and thus additional nutrients enter the 
system from deeper water. Since some zooplankton and fish migrate 
vertically through both communities, there is a degree of biological transport 
between the two vertically separated environments. 


1 The marine phytoplankton community is composed of severa! diverse 
groups of algae that carry out autotrophic production and begin the pelagic 
marine food chain. Photosynthesis results in the production of high-energy 
organic materials from carbón dioxide and water plus inorganic nutrients. 

2 Photosynthesis involves a series of interrelated Chemical reactions. The 
light reactions depend upon chlorophyll and accessory pigments capturing 
photons of light, so that radiant energy is converted to Chemical energy. The 
dark reactions do not require light; they reduce the carbón dioxide and 
produce high-energy carbohydrates as end producís. Respiration in plants and 


animáis is the reverse process of photosynthesis, whereby oxygen is used to 
release the energy contained in carbohydrates and carbón dioxide is liberated. 

3 All phytoplankton species require certain inorganic substances to carry 
out photosynthesis, including sources of nitrogen, phosphorus, and iron (also 
silica for diatoms) which may be in concentrations that are low enough to be 
limiting to plant production. Some species also require certain organic 
substances (e.g. vitamins) for auxotrophic growth, and these also may be 
present in limiting concentrations. 

4 Estimates of the total phytoplankton crop (standing stock or biomass) in 
a particular locality can be determined by measurements of cell numbers, 
total volume, or most commonly, by quantity of chlorophyll a. The rate of 
primary production is most often measured by following the uptake of 
radioactive 14 C in samples of seawater containing phytoplankton. 

5 The amount of photosynthesis increases with light intensity up to a 
máximum valué known as P max which is specific for each species. When 
light intensity increases beyond this valué, the rate of photosynthesis 
declines due to photoinhibition. The light intensity at which plant 
photosynthesis (production) exactly equals plant respiration (losses) is the 
compensation intensity. Gross photosynthesis describes total photosynthesis; 
net photosynthesis is gross photosynthesis less respiratory losses. 

6 Photosynthetic responses of phytoplankton species to light can be 
described by a series of equations based on valúes for P m ax and K¡. P max 
valúes are generally higher at warmer temperatures and in eutrophic waters. 

7 Phytoplankton are exposed to differing light intensities as light changes 
over the course of a day and as the algae are mixed vertically in the surface 
layers of the sea. At the critical depth, photosynthetic gains throughout the 
water column are just balanced by respiratory losses in the phytoplankton. If 
the depth of water mixing is greater than the critical depth, no net primary 
production can take place. Net production occurs only when the critical 
depth exceeds the depth of mixing. 

8 Growth rates of phytoplankton are also controlled by the concentrations 
of essential nutrients in seawater. Oligotrophic regions have low 
concentrations of essential nutrients and therefore low productivity. Eutrophic 
waters contain high nutrients and support high numbers of phytoplankton. 

9 Each species of phytoplankton has a particular response to different 
concentrations of limiting nutrients, and each has a different máximum 
growth rate. These differences and the species-specific responses to different 
light intensities, temperatures, salinities and other parameters, mean that 
heterogeneous and fluctuating environmental conditions favour different 
species at different times and allow many species to coexist in the same 
body of water. Thus phytoplankton species diversity can be high in what 
appears superficially to be a homogeneous aqueous environment. 

10 Solar radiation and essential nutrient availability are the dominant 
physical factors controlling phytoplankton production in the sea. The amount 
of light varies with latitude, and the amount of nutrients contained in the 
euphotic zone is largely determined by physical factors controlling vertical 
mixing of water. 

11 Despite year-round high light intensity, tropical regions are generally 
low in productivity because solar heating stabilizes the water column and 


nutrients remain at low concentrations within the euphotic zone. Conversely, 
polar regions are generally high in nutrients but low in solar radiation except 
for a brief period in the summer. Máximum annual productivities are 
generally found in températe latitudes where light and nutrients are both 
reasonably abundant. 

12 The general latitudinal pattems of primary productivity are altered by a 
number of different physical processes that lead to nutrients being 
redistributed in the water column in discrete areas. These processes occur on 
scales varying from very large (e.g. gyres and continental upwelling), to 
smaller (e.g. tidal fronts and rings), to the very small scales of Langmuir 
circulation in which only the top few metres of the water column are mixed. 

13 The standing stock of phytoplankton in the surface layers of the sea 
ranges from less than 1 mg chlorophyll a m -3 to about 20 mg m 3 during a 
phytoplankton bloom. Regional oceanic primary productivity ranges from 
<50 to >600 gCm" 2 year -1 , with Coastal upwelling regions having the 
highest valúes. Total primary productivity of the world ocean is about 

40 x 10 9 tonnes of carbón per year, a figure that is approximately equivalent 
to terrestrial plant production. 

14 Zooplankton grazing removes different proportions of the phytoplankton 
production in different marine areas. Much of the plant production is 
consumed in areas where growth rates and generation times of the 
zooplankton permit tight coupling with any phytoplankton increase (e.g. 
tropical waters). Where there is a lag in the development of zooplankton 
relative to increases in phytoplankton biomass, then some of the algal 
community dies and sinks to become a food source for deeper-living pelagic 
or benthic animáis (e.g. North Atlantic). 

15 The vertical profile of phytoplankton production changes with season 
and with Iatitude. High surface productivities generally occur in températe 
latitudes in spring and autumn, whereas chlorophyll and productivity 
maxima occur considerably deeper in tropical waters. 

Now try the following questions to consolídate your understanding ofthis 

QUESTtON 3.9 1 í í s ge ñera 1 1 y coi iside red that a high ti i versi í y o f spec ies i s 
found in spatially heterogeneous environments sueh as rainforests and coral 
reefs. What are the reasons for the greal diversity of phytoplankton (see 
Table 3.1) found in the pelagic en virón merst? 

QUESTION 3.10 if a rute of photosynthesis is measured at 0.2 mg C ni h 5 
and the standing stock oí phytoplankton is 2.5 mg C m \ what is the 
doubhng time of the phytoplankton in the sample? Refer to equations 3.7 
and 3.10, 

CUESTION 3.11 Refer to Table 3.3 (a) Is the growth rale of the popa kit ion in 
Question 3JO rapid or slow? (b) Where might yon tind sueh a population 

QUESTION 3.12 If the half-saturaüon canstants iK\) for ni trate uptake were 
0.1 fiM for spec ies A and 0.5 / 1 M for spec ies R and the máximum growth 
rates (/¿ nm ) of A and R were I and 2 douhlings per day respedivdy. which 
spec i es would domínate at a n ¡trate concentraban of 0.4 /iM? This can besi 
be shown by drawing a graph. 


QlLESTfüN 3.13 Whieh icrm in the equatíoti (3.1 1 ) for nulrient upiake bv 
phytoplankton is more ¡mportant in determining high species diversity* K % 
or ¿w? 

QUEST1DN 3.14 Couid you control the type of phytoplankton that grow ¡n ilie 
sea through the iniroduction of artificial nutrió ni media? 

QUESTION 3.15 What íeaiures would aífeet the growth rales and type of 
phytoplankton living under ice in the Arcüe Ocean? 

QUESTION 3.16 In what ways can phytoplankton produce harmful effeets on 
marine animáis and hunuins? 



Figure 4.1 A ‘Bongo’ zooplankton sampler, 
consisting oí duplícate plankton nets, being 
retrieved on board a research vessel. 

The animáis making up the zooplankton are taxonomically and structurally 
diverse. They range in size from microscopio, unicellular organisms to 
jellyfish several metres in diameter (refer to Figure 1.2). Although all 
zooplankton are capable of movement, by definition none are capable of 
making their way against a current. By definition also, all 
zooplankton — indeed all animáis and some micro-organisms — are 
heterotrophic. That is, they require organic substrates (as opposed to 
inorganic ones) as sources of Chemical energy in order to synthesize body 
materials. Unlike plants, which carry out autotrophic production by utilizing 
solar energy to reduce carbón dioxide, animáis obtain carbón and other 
essential Chemicals by ingesting organic materials. Animal species differ in 
how their energy is obtained: some species are herbivores which consume 
plants; others are carnivores which are capable of eating only other animáis; 
and some species are predominantly detritivores which consume dead 
organic material. Many animáis, however, are omnivores with mixed diets 
of plant and animal material. Different types of zooplankton often are placed 
in categories which describe their diets. 

In addition to size categories and positions in food chains, zooplankton can 
be subdivided into classifications based on habitat (oceanic vs. neritic 
species; see Section 1.2) and taxonomy. They also form two categories 
depending upon the length of residency in the pelagic environment; 
holoplankton (or permanent plankton) spend their entire Ufe cycles in the 
water column, whereas meroplankton are temporary residents of the 
plankton community. The meroplankton ineludes fish eggs and fish larvae 
(the adults are nektonic), as well as the swimming larval stages of many 
benthic invertebrates such as clams, snails, barnacles, and starfish. The more 
common types of holoplankton and meroplankton are described below in 
Sections 4.2 and 4.3, respectively. 


Zooplankton larger than 200 /¿m (refer to Figure 1.2) traditionally ha ve been 
collected by towing relatively fine-mesh nets through the water column. 
Plankton nets vary in size, shape, and mesh size (Figure 4.1), but all are 
designed to capture drifting or relatively slow-moving animáis that are 
retained by the mesh. The simplest nets are conical in shape, with the wide 
mouth opening attached to a metal ring and the narrow tapered end fastened 
to a collecting jar known as the cod end. This type of net can vary in length 
and diameter and in mesh size, and it can be towed vertically, horizontally, 
or obliquely through the desired sampling depths. Such a net will filter water 
and collect animáis during an entire towing period. More sophisticated nets 
are equipped to be opened and closed at selected depth intervals, and a series 
of such nets may be attached to a single frame to allow sampling of different 
discrete depths during a single towing operation. Analyses of the collected 
samples permit a more detailed picture of the vertical distribution of 
zooplankton within a particular area. Because many zooplankton migrate 
vertically during each 24-hour period, the time of sampling is also critical in 
tracking these changes in depth distribution. 


The selection of a particular net depends upon the type of organisms desired 
and the characteristics of the water being sampled. For example, a fine-mesh 
net (with a mesh opening of ca. 100-200 (i m) obviously must be used to 
collect small mesozooplankton. However, the same net is not suitable for 
sampling larger, relatively fast-swimming zooplankton, like fish larvae, 
because it clogs quickly and must be towed slowly to avoid tearing. ín deep 
water, where zooplankton tend to be larger and are less abundant, it is 
common to use a very large coarse-mesh net. All nets can be equipped with 
a flowmeter that estimates the total volume of water filtered during a tow; 
this permits a quantitative representation of the zooplankton collected. The 
newest towed samplers, such as the Batfish shown in Figure 4.2, 
simultaneously measure salinity, temperature, depth, and chlorophyll a 
concentration while counting and sizing zooplankton that pass through an 
optical sensor. Such devices can be set to undulate over a set depth range, 
thus making it possible to obtain samples at several depths. 

No single sampler is capable of capturing all zooplankton within its path. 
Zooplankton smaller than 200 ¡im (nano- and microplankton) cannot be 
satisfactorily sampled in nets; instead, a known volume of water is collected 
in sampling bottles or by pumps from defined depths and the smallest 
zooplankton are concentrated and removed by filtration, centrifuging, or 
settling and sedimentation. Both planktonic protozoans and phytoplankton 
can be counted in the concentrated water samples. 

Figure 4.2 A towed Batfish plankton sampler which simultaneously estimates phytoplankton 
and zooplankton abundance while recording environmental parameters. F, fluorometer for 
detecting chlorophyll a; L, light sensor; 0PC, optical zooplankton counter; Pl, intake for 
zooplankton sampling; SB, stabilizer; STD, salinity—temperature—depth sensor; 

T, towing arm. 


Figure 4.3 A scuba diver counting the number 
of zooplankton (larvacea) contained within a 
frame of known dimensions. 

Further, it is now recognized that some zooplankton are capable of avoiding 
towed samplers, which they detect either visually or by sensing turbulence 
created in advanee of the moving gear. In addition, certain gelatinous 
plankton are so fragüe that they are impossible to collect intact in nets; 
others disintegrate rapidly in the preservatives that are routinely used to store 
collections. Crustaceans usually comprise the majority of zooplankton in net 
collections because most are too small to avoid capture, and because their 
hard exoskeletons protect them from damage and distortion in nets and 
preservatives. Thus the numbers of Crustácea relative to other types of 
zooplankton may be overestimated, and net-collected plankton may not 
provide a true representation of the natural plankton community in many 
areas. The numerical dominance and biomass contribution of crustaceans, 
especially copepods, needs to be reassessed in many localities. 

Zooplankton can also be observed directly in the field, either by scuba 
diving down to depths of about 30 m (Figure 4.3) or, in deeper waters, by 
using manned submersibles or ROVs (remotely operated vehicles) which are 
tethered to ships and coupled with underwater video cameras of high 
resolution. These techniques have resulted in discoveries of new species, 
particularly of fragüe forms; in an awareness of the problem of 
underestimating numbers and biomass of these animáis from net collections; 
and in new behavioural observations of many species. Bioacoustic methods, 
developed from the use of sonar to lócate fish schools, are also being applied 
to lócate and estimate densities of larger zooplankton, such as euphausiids, 
which form dense aggregations. 


There are approximately 5000 described species of holoplanktonic 
zooplankton (excluding protozoans) representing many different taxonomic 
groups of invertebrates (Table 4.1). Those groups that are commonly found 
in the sea and that form significant fractions of the plankton community are 
described below. In addition to providing descriptive anatomical accounts, 
particular attention is given to describing the food and feeding mechanisms 
of each group as these are important in the discussions of food webs and 
energy transfer which follow. 

The smallest of the zooplankton are certain unicellular protists (Table 4.1). 
Included are many species of dinoflagellates that are partly or wholly 
heterotrophic (see Section 3.1.2 for a discussion of autotrophic species). 
These heterotrophic dinoflagellates feed on bacteria, diatoms, other 
flagellates, and ciliate protozoans that are either drawn to the predator by 
flagella-generated water currents, or that are trapped in cytoplasmic 
extensions of the dinoflagellate. Some species are only capable of 
functioning as heterotrophs; others that contain chloroplasts may also 
function as autotrophs part of the time. The best known heterotrophic 
dinoflagellate is Noctiluca scintillans (Figure 4.4), which has the form of a 
gelatinous sphere 1 mm or more in diameter. Noctiluca often occurs in 
dense swarms near coasts, and it feeds on small zooplankton (including fish 
eggs) as well as on diatoms and other phytoplankton. 

A taxonomically diverse group of flagellated protists, commonly called 
zooflagellates, ineludes all those species that are colourless and strictly 
heterotrophic. All of the organisms in this group lack chloroplasts and plant 

Table 4.1 Major taxonomic groups and representativas of holoplanktonic 




Common genera 




(= heterotrophic 









Strombidium; Fave lia 



Aglantha; Cyanea 



Physalia; Nanomia 






Be roe 










Limacina; Clio 




(Class Crustácea) 


Evadne; Podon 




Calañas; Oithona 








Sergestes; Lucifer 





Salpa; Pyrosoma 

pigments, and many feed on bacteria and detritus. Although they are very 
small (typically 2-5 ¡jl m), they have potentially high reproductive rates and 
therefore can become exceedingly abundant under favourable environmental 
conditions. Heterotrophic flagellates account for 20-80% of the 
nanoplankton by cell number, and thus they may be an important food for 
zooplankton that feed on small organisms. 

Figure 4.4 Noctiluca, a heterotrophic 
dinoílagellate. (Diameter, 1 mm.) 


Figure 4.5 Hastigerina pelágica , a pIanktonic 

foraminiferan. A bubble capsule of cytoplasm 
surrounds a central shell with numerous 
radiating spines. Fine strands of cytoplasm, 
known as rhizopodia, project along the spines. 
The rhizopodia have sticky surfaces and are 
used to snare prey such as small copepods and 
a variety of microzooplankton. (Diameter is 
approximately 3 mm.) 

Marine amoebae inelude the Foraminifera (Figure 4.5), which are 
characterized by having a calcareous perforated shell, or test, that is usually 
composed of a series of chambers. The size range of planktonic species is 
about 30 /xm to a few millimetres. Food, consisting of bacteria, 
phytoplankton or small zooplankton, is captured by specialized slender 
pseudopodia (called rhizopodia) that project through the pores of the test. 
Although there are less than 40 known planktonic species (but ca. 4000 
benthic species), these holoplanktonic foraminifera are very abundant, 
particularly between 40°N and 40°S where they generally inhabit the top 
1000 m of the water column. After death, the shells of these protozoans sink 
and accumulate in large quantities on the seafloor, forming a sediment 
known as foraminiferan ooze. 

The Radiolaria (Figures 1.7, 4.6 and Colour Píate 9) are spherical, 
amoeboid protozoans with a central, perforated capsule composed of silica. 
Most are omnivorous, and they have branched pseudopodia (called 
axopodia) for food capture; prey ineludes bacteria, other protists, and tiny 
crustaceans as well as phytoplankton (especially diatoms). The size of 
individual organisms ranges from about 50 /xm to as much as several 
millimetres in diameter; some species form gelatinous colonies composed of 
many individuáis and up to a metre or so in length. Radiolaria occur in all 
oceanic regions but are especially common in coid waters, and many are 
deep-sea species. A sediment composed of the siliceous remains of these 
protozoans is called radiolarian ooze. 

Planktonic ciliates (Colour Píate 10) are present in all marine regions and 
are often extremely abundant. All use cilia for locomotion, and some have 
modified oral cilia used for food capture. Ciliates can feed on small phyto- 
and zooflagellates, small diatoms, and bacteria. Tintinnids (Colour Píate 11) 
make up one large subgroup (>1000 species) of marine ciliates. They are 
noted for their vase-like external shells that are composed of protein; 
because this substance is biodegradable, the shells are not present in 
sediments. Despite their small size (about 20-640 /xm), tintinnids are of 
considerable ecological significance as they are widely distributed in both 


Figure 4.6 A large radiolarian with a central spherical skeleton composed of silica. 
Numerous axopodia radíate from the central capsule of cytoplasm that lies within the 
sponge-like skeleton. Prey such as tintinnids, small copepods and other microplankton are 
captured by the sticky surfaces of the axopodia. Many of the white spots in the photo are 
algae that typically live in association with this radiolarian. (Size is approximately 1 mm.) 

open seas and Coastal waters, where they feed primarily on nanoplanktonic 
diatoms and photosynthetic flagellates. In Coastal waters, tintinnids may 
consume 4-60% of the phytoplankton production. In turn, they are prey for 
a wide variety of mesozooplankton. 

Jellyfish, or medusae (Colour Píate 12), are conspicuous and common 
inhabitants of both the open sea and coastal waters. Some species are 
holoplanktonic, but others have an asexual benthic stage in their Ufe cycle and 
thus their medusae are part of the meroplankton. Although jellyfish belong to 
several different taxonomic groups within the Phylum Cnidaria, all are 
characterized by a primitive structural organization, and all are carnivorous, 
capturing a variety of zooplanktonic prey by tentacles equipped with stinging 
cells called nematocysts. They range in diameter from just a few millimetres 
to 2 m for Cyanea capillata , a common northern species with 800 or more 
30-60 m-long tentacles. Some well-known pelagic Cnidaria are colonial 
forms, like siphonophores (Figure 4.7 and Colour Plates 13 and 22), in 
which many individuáis with specialized functions are united to form the 
whole organism. Physalia , or the Portuguese man-of-war, is a tropical 
siphonophore that floats at the surface with its tentacles extending as far as 
10 m below; it is capable of capturing sizeable fish, and its stings can be 
painful to swimmers. However, the medusae known as box jellyfish are much 
more dangerous. Chironex fleckeri of tropical Australia is the most venomous 
animal on Earth, and this ‘sea wasp’ has caused at least 65 human fatalities in 
the last century. The stings of a large individual, with up to 60 tentacles 
stretching some 5 m, can cause death within four minutes. In nature, Chironex 
uses this potent venom to quickly kill prey such as shrimp. 


Figure 4.7 Siphonophores: (a) a surface floating species, Physalia physalis, the Portuguese 
man-of-war, with tentacles up to 10 m long; (b) a swimming species, Nanomia sp., ca. 

10 cm long. Both species use their long trailing tentacles to capture prey. 

Ctenophores are closely related to jellyfish, but their structure is sufficiently 
different to warrant their being placed in a sepárate phylum. These are 
transparent animáis that swim by means of fused cilia arranged in eight rows 
(called comb plates). Like the Cnidaria, ctenophores are carnivores, but they 
lack the nematocysts of their cióse relatives. Certain of the ctenophores like 
Pleurobrachia (Colour Píate 14) have long paired tentacles with adhesive 
cells that are used to capture prey; other species (e.g. Bolinopsis) capture 
food in large ciliated oral lobes. These ctenophores can have significant 
impacts on fish populations as they feed directly on fish eggs and fish larvae, 
and they also compete with young ñsh for smaller zooplankton prey such as 
copepods. Some ctenophores like Beroe (Colour Píate 15) lack tentacles but 
have large mouths; they engulf their prey, which consists principally of 
tentaculate ctenophore species. 

Chaetognaths, or arrow worms, (Figure 4.8), are one of the best known and 
most abundant carnivorous planktonic groups. These hermaphroditic animáis 
are found only in the sea, down to depths of several thousand metres. They 
have transparent, elongate and streamlined bodies, and most are less than 
4 cm long. They often remain motionless in the water, but are capable of 
swift darting motions when in pursuit of prey. Food, consisting of smaller 
zooplankton, is captured by clusters of chitinous hooks located around the 
mouth of the predator. Chaetognaths do not seem to be selective in prey 
type; often the type of food eaten reflects local relative abundance of suitable 
prey. There are a few other holoplanktonic worms belonging to different 
phyla but, in most regions, these are generally found in very low numbers. 


Figure 4.8 The chaetognaths (a) Sagitta 
pulchra and (b) Sagitta ferox. 



Figure 4.9 The planktonic polychaete 
Tomopteris helgolandica with múltiple swimming 
legs and long, slender, paired antennae. Length, 
45 mm. 

One exception is the polychaete (Phylum Annelida) genus Tomopteris 
(Figure 4.9) with about 40 species (all predators) distributed throughout the 
world ocean. 

•UEST10N 4.1 Chaetognaths, ctenophores. sonie jellytish» and many oiher 
zooplankton living in the upper layéis of the sea are transparent, Can yon 
suggest whv iranspareney is a use ful characteristic for animáis living in the 
euphotic /one? 

Only a few species belonging to the Phylum Mollusca have become 
holoplanktonic. Heteropods are a small group ( ca. 30 species) of molluscs 
that are closely related to snails, but these pelagic forms swim by undulating 
motions of a single fin developed from the creeping foot of their benthic 


ancestors. Some of the species can completely withdraw into a small 
(<10 mm) external, spirally-coiled shell (Colour Píate 16); others have 
reduced shells or lack shells entirely and are highly transparent animáis that 
may attain lengths of up to 50 cm (Colour Píate 17). Despite these external 
differences, all heteropods have remarkably well-developed eyes and are 
visual predators, feeding on other planktonic molluscs, or on copepods, 
chaetognaths, salps, or siphonophores. Prey are actively pursued and captured 
by large chitinous teeth which can be protruded from the mouth. Heteropods 
are generally found in oceanic warm water areas, but, like many carnivorous 
plankton, they are not very abundant in any given locality. Their calcium 
carbonate shells are sometimes found in sediments on the ocean floor. 

The shelled pteropods, or thecosomes, are also holoplanktonic snails. Most 
of them have a thin, external, calcareous shell measuring from a few to 
about 30 mm in largest dimensión. The shell is spirally coiled in primitive 
species but assumes a variety of shapes in more advanced members (Colour 
Plates 18, 19). One subgroup (called pseudothecosomes) (Colour Píate 20) is 
made up of larger animáis (>30 mm) that lack a true shell; instead, there is 
a cartilaginous, internal, skeletal structure. All thecosomes swim by means 
of paired wings or a fused wingplate, structures that developed from the foot 
of benthic molluscan ancestors. Despite considerable structural diversity, all 
thecosomes are suspensión feeders. They produce large, external, mucous 
webs that are held in the water while the animal remains motionless below. 
As the web filis with organisms entangled in the sticky strands of mucus, it 
is withdrawn and ingested. Food consists of phytoplankton as well as small 
zooplankton and detrital material. Some of the shelled pteropods can be very 
abundant in epipelagic areas, particularly those species that inhabit polar 
seas. Some thecosomes are an important food source for pelagic fish, 
including commercially important species like mackerel, herring, and salmón. 
The shelled pteropods have an unusual reproductive pattern in which an 
animal is first a male that mates with another male; the sperm is stored until 
the animal changes into a female that lays fertilized eggs in mucoid floating 
masses. The carbonate shells of dead animáis eventually sink and accumulate 
in certain areas to form a type of sediment known as pteropod ooze. 

Thecosomes are preyed upon by another group of planktonic gastropod 
molluscs, the naked pteropods or gymnosomes. These animáis lack shells as 
adults, but they too swim by means of paired wings. Of the approximately 
50 species, Clione limacina (Colour Píate 21) is the largest (to 85 mm long), 
most abundant, and best known. It lives in polar and subpolar regions of the 
Northern Hemisphere, where it feeds only on several species of the 
thecosome genus Limacina (Colour Píate 18). Other gymnosomes also are 
predators that feed exclusively on specific shelled pteropods. All capture 
prey with special tentacles and chitinous hooks, and they remove the soft 
parts of the prey from its shell before swallowing it. 

The segmented Crustácea are represented in the sea by several different 
groups, but copepods are the predominant forms. Some of the most 
abundant and best known marine zooplankton belong to the Order 
Calanoida (Figure 4.10a-c) which comprises about 1850 species. These 
free-living calanoid copepods are present in all marine regions and usually 
make up 70% or more of all net-collected plankton. All species have three 
distinctive body regions: the head and first segment of the body are fused 
and bear two pairs of antennae and four pairs of mouthparts; the segmented 
mid-body has paired swimming legs; and the narrow posterior section lacks 


1 mm 

Figure 4.10 Planktonic Crustácea. Calanoid 
copepods: (a), Pseudocalanus eiongatus: (b), 
Centropages typicus ; (c), Calanus finmarchicus. 
(d), Microsetelía norvegica, a harpacticoid 
copepod. (e), Oithona simi/is, a cyclopoid 
copepod. Cladocera: (f), Podon leuckartr, (g), 
Evadne nordmanni. (h), Conchoecia eiegans, an 
ostracod. Euphausiids: (i), Thysanoessa inermis ; 
(j), Meganyctiphanes norvegica. (k), Themisto 
abyssorum, an amphipod. (I), Gnathophausia 
zoea , a mysid. 

appendages. Total body length is usually less than 6 mm, but some 
exceptional species exceed 10 mm in length. Many of the species feed by 
capturing phytoplankton, especially diatoms, in currents generated by 
movements of the swimming legs and mouthparts. Some calanoids, however, 
are omnivorous or carnivorous and feed on small zooplankton. The sexes are 
sepárate, and fertilized eggs may be laid freely in the water or may be 
retained in an external cluster by the female. Development involves twelve 
different stages, each separated by moulting, or casting off of the 
exoskeleton, and marked by the appearance of new segments and additional 
appendages. The first six stages are nauplius (plural, nauplii) larvae 
(designated NI to NVI); the last six are copepodite stages (CI to CVI), with 
CVI being the sexually mature adult. 

Another copepod group, the Order Cyclopoida (Figure 4.10e), differs in that 
members have relatively shortened antennae and more segments in the 


posterior third of the body. The order contains over 1000 species, but the 
majority live among benthic algae or in bottom sediments; only about 250 
species are planktonic. Small species belonging to the planktonic genera 
Oithona and Oncaea can, however, be very abundant. Some of the 
cyclopoid copepods have specially modified antennae for capturing 
individual microzooplankton. 

The majority of copepods belonging to the Order Harpacticoida 
(Figure 4.10d) are Coastal or live in association with the sea bottom. 
Approximately 20 species are holoplanktonic, and these are characterized by 
usually being very small (<1 mm long) and without distinct divisions 
between body regions. Although some species are widely distributed and 
may be seasonally or locally abundant, their ecological importance in the 
plankton community does not seem to be great. 

The euphausiids (Figure 4. lOi-j) form another important group of marine 
Crustácea with 86 species. These shrimp-like animáis are generally of 
relatively large size, with many species attaining a length between 15 mm 
and 20 mm and with some exceeding 100 mm. Euphausia superba is the 
krill of the Antarctic Ocean, where this abundant species is a major 
component of the diet of many larger animáis and is itself harvested 
commercially (see Sections 5.2 and 6.1). Euphausiids are fast-swimming and 
are usually undersampled by large nets because of their visual perception and 
avoidance capabilities. But it is known that euphausiids form major fractions 
of the zooplankton biomass in the open ocean of the North Pacific and North 
Atlantic and in the Arctic, and they are important food for fish (e.g. herring, 
mackerel, salmón, sardines, and tuna) and some seabirds in these areas. 
Euphausiids are generally omnivorous with food consisting of detritus, 
phytoplankton, and a variety of smaller zooplankton. Larger species are also 
capable of feeding on fish larvae. Euphausiids, like copepods, have a series 
of anatomically distinct larval stages separated by moulting and growth. 

Amphipods (Figure 4.10k) are distinguished from other Crustácea by having 
laterally compressed bodies. They usually constitute only a small fraction of 
the total zooplankton. Parathemisto gaudichaudi is a common pelagic 
species with a wide distribution at relatively high latitudes in both 
hemispheres. The adults of this species are free-living carnivores, feeding on 
copepods, chaetognaths, euphausiids, and fish larvae. Many pelagic 
amphipods, however, are commonly found attached to siphonophores 
(Colour Píate 22), medusae, ctenophores, or salps, and the amphipods either 
feed as predators on these animáis or act as parasites. In contrast to 
copepods and euphausiids, amphipods have direct development; the young 
are released from a brood pouch and look like miniature adults. 

Ostracods (Figure 4.1 Oh) are usually minor components of the zooplankton 
community. These crustaceans have a unique, hinged, bivalved exoskeleton 
into which the animal can withdraw. Most species are rather small although 
Gigantocypris , a deep-water inhabitant, reaches more than 20 mm in 
diameter. Little work has been done on feeding habits in this group, but 
some species are regarded as scavengers. 

Although there are over 400 species of freshwater Cladocera (including 
Daphnia , the common water flea), there are only about eight marine species 
(Figure 4.10f-g) in this primitive group of crustaceans. The marine 
cladocerans are primarily of interest in Coastal and brackish water, although 
there are species that become seasonally abundant for brief periods in the 


open ocean. Because they are capable of producing cloned offspring by 
parthenogenesis (i.e. reproduction without males and without fertilization), 
Cladocera are able to rapidly increase their numbers when environmental 
conditions are favourable. 

Mysids (Figure 4.10/) are Usted in Table 4.1 and mentioned here for 
completeness, but they seldom are important components of the plankton 
community. Many of these shrimp-like animáis spend part of the time on the 
seafloor, but rise into the overlying water at night or when forming breeding 
swarms. A few oceanic species are residents of near-surface waters, but most 
Uve in deeper zones. The most abundant and best known species are 
estuarine or inshore residents, and some of these are harvested commercially 
in parts of Asia. 

The most advanced Crustácea are decapods, a group that encompasses 
shrimp, lobsters, and crabs. Most are benthic, but some are holoplanktonic or 
nektonic. Pelagic species inelude about 210 species of shrimps (Colour 
Píate 23) that typically measure 10-100 mm or more in length and thus 
constitute some of the larger zooplankton. They are strong swimmers and 
avoid capture by ordinary plankton nets. Most Uve below 150 m depth in 
daytime. They are usually omnivores or predators, and copepods, 
euphausiids and other planktonic Crustácea are their predominant foods. 
Densities of pelagic shrimp in oceanic areas are typically on the order of 1 
individual per 200-2000 m 3 of water, but they can be important prey for 
various fish, including albacore tuna, and for dolphins and whales. 

Two groups of chordates are important members of marine zooplankton 
communities. Appendicularians (Colour Píate 24) are closely related to 
benthic tunicates or sea squirts (see Section 7.2.1). Because they closely 
resemble the larval stages of these bottom-dwelling relatives, they are also 
known as larvaceans. The body of appendicularians looks like a tadpole; it 
consists of a large rounded trunk containing all the major organs and a 
longer, muscular tail. Most species secrete a spherical balloon of mucus, 
called a house, in which they reside. The body is generally only a few 
millimetres long, whereas houses range from about 5 mm to 40 mm long. 
Movements of the animal’s tail create a current of water that enters the 
mucoid house through mesh-covered filters which remove larger particles of 
suspended material. As water flows through the house, it passes another 
feeding filter where nanoplankton and bacteria are collected and transported 
to the mouth. Periodically the filters become clogged with particles and the 
house must be abandoned, an activity that can be repeated up to a dozen 
times per day; new houses can be secreted within a few minutes. Discarded 
houses can reach densities of more than 1000 itT 3 , and they contribute to 
the formation of marine snow, a term applied to macroscopic aggregates of 
amorphous particulate material derived from living organisms. Abandoned 
houses represent rich sources of food and surfaces for attachment by other 
organisms in the water column and, for these reasons, they are rapidly 
colonized by bacteria and protozoans. Larvaceans grow rapidly, and have 
short generation times of 1-3 weeks. They are among the most common 
members of the zooplankton, being especially abundant in Coastal waters and 
over continental shelves where densities may reach 5000 m~ 3 . The 70 or so 
species are distributed in all the oceans. 

Salps (Colour Plates 25, 26) constitute another class of chordates, but these 
animáis are commonly found only in warm surface or near-surface waters. 
Each individual salp has a cylindrical, gelatinous body with openings at each 


solitary generation 

Figure 4.11 The life cycle of salps. one individual oí the Chain 

end. Locomotion is achieved by muscular pumping which propels water 
through the body. This water current also brings food partióles into contact 
with an intemal net of mucus that is continuously secreted by the animal. 
Cilia transport food entrapped in mucus to the esophagus where it is 
ingested. Food consists primarily of phytoplankton and bacteria, ranging in 
size from about 1 /xm to 1 mm. Because salps often form dense swarms and 
have high feeding rates, their feeding activities may significantly reduce the 
concentrations of small-sized organisms in the surrounding water. Salps have 
an unusual life cycle (Figure 4.11) in which sexual reproduction altemates 
with asexual budding. Each species of salp has two different forms: the 
asexual form is a solitary individual (1-30 cm long) that buds to produce a 
chain of up to several hundred individuáis and as long as 15 m; each 
aggregate in the released chain is an hermaphrodite and will produce both 
sperm and a single egg. Self-fertilization does not usually occur because the 
egg and sperm ripen at different times. Following cross-fertilization, the 
single embryo grows within the parent and eventually breaks through the 
parental body wall to become a young, free-swimming, solitary individual 
which once again will continué the cycle with asexual budding. Salps and 
the related appendicularians are good examples of r-selected species (see 
Section 1.3.1) with extremely rapid growth rates and short life spans; thus 
they can quickly respond to favourable environmental conditions by 
producing large populations. 

QUESTION 4,2 Can you think of any reasons why salps may have developed a 
complex üfe cycle that involves two different reproductive pattems? 


Some benthic marine invertebrates have no free-swimming larval stage. Their 
young hatch as miniature adults from eggs attached to the sea bottom, or 
emerge directly from the parent. But approximately 70% of benthic species 
release eggs or embryos into the water column, and the resulting larvae 
become part of the plankton community. Depending on the species, these 
meroplanktonic larvae may spend from a few minutes to several months (or 


even years in exceptional cases) in the plankton before they settle onto a 
substrate and metamorphose into the adult form. During this time, the larvae 
drift in currents and may be dispersed away from the parent population. 

Some of the more common types of meroplanktonic larvae of benthic 
invertebrates are illustrated in Figure 4.12. Benthic snails and clams produce 
a shelled veliger larva that has a distinctive ciliated membrane (called a 
velum) that is used for locomotion as well as food collection. Sessile 
bamacles have free-swimming nauplius stages, usually six, which are 
similar to the nauplii of copepods and other planktonic Crustácea, but with 
characteristic pointed projections on the anterior edges of the exoskeleton; 
these naupliar stages are succeeded by a cypris which attaches to a substrate 
and metamorphoses to the adult. Starfish, sea urchins, sea cucumbers, and 

Figure 4.12 Meroplanktonic larvae of benthic 
invertebrates. (a) snail veliger; (b) polychaete 
trochophore; (c) late larva of a polychaete; 

(d) bipinnaria of a starfish; (e) echinopluteus of 
a sea urchin; (f) barnacle nauplius; (g) barnacle 
cypris; (h) crab zoea; (i) crab megalopa. 



cilia on 

edge of velum . 


0,25 mm bands 


ciliary band 

0.1 mm 

0.1 mm 




larval arms 
0.05 mm 


other benthic echinoderms have various types of meroplanktonic larvae, 
some of which are shown in Figure 4.12. Benthic worms belonging to 
different phyla also have distinctive larvae: polychaetes, for example, 
produce a trochophore larva, with several bands of cilia, that eventually 
develops a segmented body and appendages before settlement. Benthic 
decapods, like crabs, typically have a succession of different planktonic 
larval stages that are separated by moulting. Crabs usually hatch as a spiny 
zoea, which eventually changes before settlement to a megalopa that 
resembles a miniature adult. These examples are but a few of the more 
common types of larvae that appear temporarily in the plankton community. 

Benthic invertebrates living in shallower zones often produce planktonic 
larval stages, but deep-sea species commonly lack a planktonic stage and 
instead have direct development or brood protection of young. This may be 
related to a lack of suitable and abundant suspended food for planktonic 
larvae in deep water. In températe and cold-water inshore regions, 
meroplanktonic larvae of benthic invertebrates typically appear seasonally in 
response to warmer temperatures and increased phytoplankton. In tropical 
waters, the reproduction of benthic invertebrates may be more or less 
continuous but with peaks of reproductive activity tied to other 
environmental events, such as rainfall; in such areas, meroplanktonic larvae 
may be present throughout the year, but in differing abundances. 

Fish eggs and fish larvae (Figure 4.13) also form an important part of the 
meroplankton; they are referred to as the ichthyoplankton. Some fish 


t mm 

Figure 4.13 Meroplanktonic fish eggs and fish larvae. (a) anchovy egg; (b) mackerel egg; 
(c) myctophid (lantern-fish) egg; (d) cod embryo developing in egg; (e) newly hatched cod 
larva; (f) newly-hatched pilchard. 


species attach their eggs to substrates; salmonids, for example, typically 
place eggs in the gravel of streams and herring deposit eggs on seaweeds or 
directly on the seafloor. Many marine fish, however, release free-fioating, 
planktonic eggs; these inelude sardines, anchovy, tuna, and many other 
commercially harvested species. These planktonic eggs are typically 
spherical, transparent, and small, usually of the order of 1 -2 mm in 
diameter. The eggs contain varying amounts of clear yolk which is the food 
of the developing embryos and newly hatched larvae. Some eggs also 
contain oil globules, apparently to aid in fíotation. 

As with the meroplanktonic larvae of benthic invertebrates, the appearance 
of fish eggs in the plankton is dependent on the spawning eyeles of the 
adults, and these are often linked with environmental change. Rate of 
development within the egg is species-specific but is also closely tied to 
ambient sea temperatures, with hatching being delayed in colder waters. 
Hatching generally occurs within a few days to a few weeks after the eggs 
are spawned. The numbers of eggs produced may be very large. Each female 
plaice, for example, produces about 250000 eggs; haddock lay about 
500000; and cod lay over one million. If these figures are multiplied by the 
number of spawning fish, the annual production of eggs may be enormous. 
Numbers of pilchard eggs in the English Channel alone have been estimated 
to exceed 4 x 10 14 . Obviously few of the young from these eggs survive to 
adulthood; many form an important food source for the holoplankton and 
also for adult fish. 

In general, for both lisli and benthic i n vertebrales, ihere is an in verse 
córrela t ton between egg size and the numbers of eggs produced. Is the re any 
selective advantage to produdng large numbers of small eggs or, 
alternadvely, to producing small numbers of large eggs? 

Two life patterns are operable with regard to egg size. Some species produce 
large eggs, but in small numbers because of size and energy restrictions. 
Larger eggs contain more yolk and the newly hatched young will be larger 
relative to young from smaller eggs. Larger young tend to have higher 
survival rates, probably because they are already too large to be eaten by 
some pelagic predators, and possibly because they are also more active and 
better able to evade potential predators. On the other hand, many species 
produce enormous numbers of small eggs which contain little or no nutritive 
material for the developing embryos. These young hatch at a small size; they 
are vulnerable to many predators, and it is critical that they immediately 
begin feeding. Mortality is much higher for these young compared to those 
from large eggs, but the high mortality is compensated for by larger numbers. 

For the first few days after hatching, fish larvae retain remnants of the yolk 
in a sac carried under their body, and they continué to rely on this yolk sac 
as a food source until the mouth and gut develop. When the yolk is 
exhausted and the larvae begin to feed, they are totally dependent upon 
suitable food being plentifully available in the plankton. This dependeney 
lasts throughout their planktonic life, for up to several months, until they are 
large enough to be classified as nekton and can actively seek feeding areas 
independently of current drift. At the same time, the young are vulnerable to 
pelagic predators, both larger zooplankton and nekton. Mortality is high 
during the planktonic stages; usually only a small fraction of fish survive to 
adulthood. For example, it is estimated that early life mortality in cod is as 
much as 99.999%. 


We have previously classified zooplankton according to size, habitat, 
taxonomic position, and length of residency in the plankton community. 
Zooplankton are also grouped according to their depth position in the 
water column. 

Those species that live permanently at the sea surface and whose bodies 
project partly into the air are called pleuston. They are sometimes 
considered to form a special category because they are passively transported 
by wind instead of by currents. The neuston ineludes those species that 
inhabit the uppermost few to tens of millimetres of the surface water. 
Ecologically, it is difficult to sepárate these categories, and here they are 
discussed together as organisms living in the uppermost zone of the ocean. 
This community is richly developed in tropical waters, and most of the 
foliowing examples are typically warm water species. 

Examples of pleustonic species inelude the colonial cnidarians Physalia 
(Figure 4.7a) and Velella (Colour Píate 27) and their relatives, all of which 
have gas-filled floats that project above the water surface. The long trailing 
tentacles of Physalia enable it to capture zooplankton and small fish well 
below the sea surface. Velella has short tentacles and its food (copepods, 
larval crustaceans, and eggs of fish and euphausiids) is captured from very 
near the surface. Despite their stinging tentacles, sea turtles and several 
surface-dwelling molluscs feed on these Cnidaria. Janthina (Colour Píate 28) 
is a snail that builds a raft of air bubbles encased in mucus; the animal hangs 
suspended, upside-down, from this float at the sea surface. It feeds on both 
Physalia and Velella as well as on other neustonic animáis. Glaucus (Colour 
Píate 29) is a nudibranch (or sea slug) that floats upside-down at the surface 
by ingesting air which it then stores in special sacs of the digestive tract. It 
too feeds on both Physalia and Velella , and it ingests the nematocysts along 
with other tissues. Glaucus has the remarkable ability to absorb the stinging 
cells without causing them to discharge, and the nematocysts are then 
conveyed by cilia to special sacs in the tips of extemal papillae on the 
nudibranch’s dorsal surface. There, they can be employed as a defence 
against predators, and they will discharge if the animal is disturbed. Bathers 
who come in contact with Glaucus which have washed ashore report painful 
stings lasting several hours. Other permanent members of the neuston 
community inelude the only inseets found in the open ocean; they belong to 
the insect order Hemiptera and the genus Halobates. These wingless water 
striders cannot survive immersion in seawater and thus are restricted to 
eating other organisms living at the immediate surface, including floating 
cnidarians and neustonic species of copepods. 

Smaller organisms are also present in the immediate surface layers of the 
sea. Bubbles produced by breaking waves accumulate organic material in a 
thin surface scum, and this provides a rich substrate for bacteria which often 
concéntrate in films. This very thin surface región of only a few millimetres 
in depth may contain from 10 to 1000 times more bacteria than in the water 
immediately below, and consequently these organisms may be an important 
part of the neuston, forming food for large numbers of protozoans such as 
tintinnids. The high light intensities inhibit photosynthesis and because few 
phytoplankton are found in this zone, the bacteria and protozoa are probably 
important components in the diets of grazing copepods. Many types of 
meroplanktonic invertebrate larvae and the eggs and larvae of certain fish 


species are also commonly present in this zone. Indeed, the eggs of some 
fish (e.g. anchovy, mullet) are extremely buoyant, adhering to the surface 
film, and the upper few centimetres of the sea support many different types 
of fish larvae. Some of these fish remain at all times near the surface, but 
others — as well as some other types of zooplankton — are transient 
members of the neuston, usually moving to the surface zone at night to feed. 
The accumulation of organisms at the sea-air interface creates an important 
feeding zone for oceanic birds (e.g. petrels, skimmers, fulmars), many of 
which have bilis adapted for skimming this layer (see Section 6.5 and 
Figure 6.2). 

What special environmental fe atures are presern at the sea air interface to 
which neuston ic species niust adapt? 

This región receives very high levels of infrared and ultraviolet light, the 
latter being detrimental to many organisms. The high light intensities also 
inhibit the production of phytoplankton, and consequently few, if any, of the 
zooplankton residing in this zone are strict herbivores. In calm weather, this 
región may experience the most extreme 24-hour changes in temperature and 
salinity, although mixing usually produces a temperature-salinity regime that 
is indistinguishable from that of the upper 2-3 m. Permanent inhabitants of 
this zone are also exposed to extreme wave action during storms. And 
neustonic animáis, like intertidal species, are exposed to both marine and 
aerial predators. 

Neustonic animáis live in a región of high light intensity. What types of 
defences might they have against predators that hLint by sight? 

Some of the neuston, especially crab and fish larvae, are highly transparent 
and therefore difficult to discern by sight. But many tropical neustonic 
animáis (e.g. Velella, Halobates , Janthina, Glaucus ) are strikingly coloured 
with violet or blue pigments. It has been suggested that these colours may 
provide protection from the high levels of ultraviolet light, but such 
coloration also may be effective as camouflage from predators because it 
blends closely with the blue colour of tropical oceanic water. Janthina and 
Glaucus also exhibit counter-shading; that is, those body surfaces that are 
directed downward in the water are lighter in colour than areas cióse to the 
surface, and this too may be a predator defence. Marine predators like fish, 
which approach from below, would view the lighter undersides of these 
molluscs against a background of lightly coloured sky whereas, to aerial 
predators, the darker blue upper surfaces of prey would blend with the dark 
blue colour of the water. Cryptic coloration is not the only defence 
mechanism of neuston. For example, some neustonic copepods have 
developed the ability to jump out of the water in response to predators. 

A special surface community has developed in the Sargasso Sea. Sargassum , 
a floating seaweed, forms an extensive habitat for an unique association of 
more than 50 species of animáis. The total wet weight of Sargassum in the 
community has been estimated at between 4 and 10 million tonnes. Many of 
the animáis found living in the seaweed are primarily benthic, and these 
inelude hydroids, sea anemones, crabs, shrimp, and other Crustácea. Some of 
the endemic species (that is, those restricted to this particular habitat), 
including some of the crabs and fish, have developed special camouflage 
protection by coming to resemble Sargassum weed both in shape and colour. 


In all areas, the región immediately below the sea surface and extending to 
200 or 300 m is referred to as the epipelagic zone (Figure 1.1). Many 
zooplankton are permanent residents of this zone, others migrate into this 
región at night. Only the zooplankton that live in depths shallower than 
300 m during the daytime are regarded as truly epipelagic. The epipelagic 
zone coincides with the euphotic and disphotic zones (see Section 2.1.2 and 
Figure 2.5), and it supports a great diversity and abundance of life. Many 
herbivores and omnivores inhabit this región; these inelude smaller 
crustaceans (such as copepods), the thecosomatous pteropods, salps, 
larvaceans, and meroplanktonic larvae. Many of the species are relatively 
small, and many are transparent. 

The mesopelagic zone (Figure 1.1) lies between the bottom of the epipelagic 
región and a depth of approximately 1000 m, and the animáis that live here 
in the daytime are called mesopelagic species. Many mesopelagic 
zooplankton tend to be larger than their epipelagic relatives. In this deep 
nonturbulent water, even the delicate-bodied, transparent, gelatinous 
zooplankton become more diverse and increase in size. For example, in situ 
observations have revealed a deep-sea larvacean ( Bathochordaeus ) that 
makes a house of 2 m in size, and siphonophores have been seen that can 
extend to 40 m in length, making them among the longest animáis known. 
Some of the animáis of this región, such as euphausiids, are at least partly 
herbivorous, and they move upward to the epipelagic zone at night to feed 
on phytoplankton. Many of the residents, however, are camivores or detritus 
feeders, feeding on larger particles. 

Particulate organic material sinking from above, especially faecal pellets, 
tends to accumulate at depths of about 400-800 m beeause of the density 
gradient (pyenoeline) that is associated with the permanent thermocline. This 
rich food source is decomposed by bacteria, but is also consumed by 
zooplankton that tend to congrégate in these regions, and the decomposition 
and animal respiration result in high levels of oxygen utilization. These 
biological activities contribute to the formation of oxygen mínimum layers, 
or zones where oxygen concentrations may fall from the normal range of 
4-6 mg l -1 to less than 2 mg l -1 , even approaching anoxic conditions in 
some areas. Physical factors also are in volved in the formation of oxygen 
minimum layers. Oxygen is replenished at the sea surface through contact 
with atmospheric oxygen, and is carried into deep areas through the sinking 
of oxygen-laden surface water. The oxygen minimum layer thus marks the 
intermediate depth of minimal physical replenishment, as well as a zone of 
high respiration. Certain species such as the ‘vampire squid’, Vampyroteuthis 
infernalis , seem to have become uniquely adapted to live permanently within 
the low-oxygen zones; other species migrate in and out of the oxygen 
minimum layers. 

Many mesopelagic animáis have developed red or black coloration. For 
example, all pelagic shrimp living below 500-700 m by day are uniformly 
bright red (Colour Píate 23), whereas those living in shallower depths are 
transparent or semi-transparent. Many mesopelagic zooplankton (and fish) 
also have larger eyes and increased sensitivity to blue-green wavelengths of 
light; these are the deepest penetrating wavelengths of solar radiation and 
also the spectrum of most bioluminescent light. 

QUESTIQN 4.3 What is ihe adaptive sigmficance of red coloration in deep-sea 


Bioluminescence refers to light produced and emitted by organisms 
themselves, and it is known in marine species of bacteria, dinoflagellates, 
many invertebrates (both pelagic and benthic), and some fish. No 
amphibians, reptiles, birds, or mammals possess this property, and only one 
freshwater invertebrate is known to be luminous. Although the phenomenon 
occurs in many shallow-living marine species, bioluminescence becomes 
increasingly important in the deep sea where it is the only source of light 
below about 1000 m. In the midwater disphotic zone, more than 90% of the 
resident species of crustaceans, gelatinous zooplankton, fish, and squid 
emit light. 

The biochemistry of bioluminescence is not completely understood and 
apparently varíes within different species. But, in general, biologically 
generated light results from the oxidation of organic compounds known as 
luciferins in the presence of the enzyme luciferase. The Chemicals in volved 
in the reaction are synthesized by living cells. However, midshipman fish 
(. Porichthys ) are known to acquire luciferin from their diet, by eating 
luminous crustaceans; in regions where these crustaceans do not occur, the 
fish are nonluminous even though they possess the enzyme luciferase. In the 
Chemical reactions of bioluminescence, the energy, instead of being released 
as heat (as occurs in most Chemical reactions), is used for the excitation of a 
product molecule, called oxyluciferin. This ‘excited’ compound (indicated by 
an asterisk below) then releases the energy as a photon, producing light. 


luciferin + 0 2 -> oxyluciferin* -> oxyluciferin + light 

The reaction shown above can be carried out in special cells called 
photocytes, or in complex organs known as photophores. 

Bioluminescence may be used for various types of communication in the 
sea, but in many species the behavioural or ecological role of the light 
signáis remains unknown. In some planktonic species, bioluminescent 
displays result when the organisms are disturbed and are therefore thought to 
be employed as a predator defence. When disturbed, some medusae, 
siphonophores, ctenophores, ostracods, and deep-sea squid shed luminescing 
tentacles or produce clouds of luminous material as apparent decoys for 
predators while the darkened animal itself swims away. In some pelagic 
species, bioluminescence may serve as a type of camouflage by acting as a 
counter-illumination system which eliminates an animal’s silhouette against 
downward penetrating daylight. Animáis with eyes may use bioluminescent 
signáis to communicate with other individuáis of their own species. 
Euphausiids, for example, may respond to luminescence in other individuáis 
to form densely crowded schools when chased by large predators, or to 
aggregate for breeding purposes. Some siphonophores and deep-sea fish 
employ bioluminescent lures to attract prey within cióse range, thus 
eliminating the need to expend energy in hunting for food. The ability to 
produce light obviously has evolved independently in many organisms, and 
it clearly serves a variety of roles. 

Bioluminescence also occurs in some of the bathypelagic species that 
inhabit the dark water layers from 1000 to 3000 or 4000 m, and in some of 
the abyssopelagic species living below these depths (Figure 1.1). In these 
zones, many of the zooplankton and fish tend to be deep red or black in 
colour and many have smaller eyes than the mesopelagic species. Away from 
the productive surface waters, there are fewer species and fewer individuáis. 


zooplankton biomass (mg nrf 3 ) 

0.01 0,1 1.0 10 100 1000 

Figure 4.14 The biomass of net-collected 
zooplankton (excluding Cnidaria and salps) from 
the surface to 8000 m depth in the Northwest 
Pacific Ocean (45°N) (black line) and in the 
tropical Pacific (6°S) (blue line) during July. 

The general decrease in zooplankton biomass with depth is shown in 
Figure 4.14 for several regions in the Pacific Ocean. On average, the 
biomass of plankton collected in nets decreases by 1-1.5 orders of 
magnitude from the surface to 1000 m, and decreases by another order of 
magnitude between 1000 m and 4000 m. This exponential decrease in 
biomass with depth is correlated generally with longer generation times and 
lower fecundity in deeper-dwelling species. 

One other category of vertical distribution has been established to encompass 
those pelagic species that live either cióse to the seafloor, or that are 
temporarily in direct contact with the sea bottom. These animáis are referred 
to either as epibenthic or demersal. This category ineludes many Crustácea, 
especially shrimp and mysids, and it also applies to bottom-dwelling fish, 
like solé and plaice. Where the water is not very deep, these species too may 
move away from the seafloor at night. 

The depth classification System given here is based on recognition that 
environmental conditions change with increasing depth in the sea, and that 
the animáis living at different depths have generally evolved different life 
strategies. However, it should be stressed that the vertical ecological zones 
described here are arbitrary regions, and that individuáis and species are not 
distributed uniformly within these depths (see Figure 4.22). Many animáis 
move between the different zones, and the deeper meso-, bathy-, and 
abyssopelagic zones are not clearly distinguishable from each other. Further, 
the depth distribution of some species may change with latitude. This is 
particularly true of those cold-water species that are tolerant of a great range 
of hydrostatic pressure. The chaetognath Eukrohnia hamata , for example, 
lives near the surface in polar areas, but is found only in deep, coid water in 
low latitudes. 


bathy pelagic 

Figure 4.15 A schematic illustration of the diel 
migration patterns of pelagic shrimps living in 
different vertical zones. Dotted and hatched 
areas indícate the depth ranges of the main day 
and night concentrations, respectively, of the 
different groups. Each group (1-7) is a 
composite of different species that occupy 
similar depth ranges. 

One of the most characteristic behavioural features of plankton is a vertical 
migration that occurs with a 24-hour periodicity. This has often been 
referred to as diurnal vertical migration. However, diurnal refers to evenís 
that occur during daytime; it is the opposite of nocturnal. Diel refers to 
events that occur with a 24-hour rhythm. Diel vertical migration (or DVM) 
is usually marked by the upward migration of organisms towards the surface 
at night, and a downward movement to deeper waters in the daytime. This 
phenomenon has been known since the time of the Challenger Expedition 
(Section 1.4), but even now we do not have entirely satisfactory explanations 
for the widespread occurrence and ecological significance of this 24-hour 
rhythmical movement. Diel vertical migration occurs in at least some species 
of all the major groups of zooplankton (freshwater species as well), and it is 
known in dinoflagellates and in many nektonic species, including both 
cephalopods and fish. Diel vertical migration occurs in many (but not all) 
epipelagic and mesopelagic species and, although few studies have been 
done on deeper-living plankton, it is known in some bathypelagic shrimp 
(Figure 4.15). 

Because of diel vertical migrations, a comparison of day and night plankton 
tows taken in the same area at the same depths will always show differences 
in species composition and total biomass. This can be seen, for example, in 


number per 1000 m 3 of water 

Figure 4.16 Day (solid lines) and night 
(dashed lines) distributions of the juveniles and 
adults of Euphausia hemigibba at one station in 
the California Current. 

Figure 4.16 which compares the day and night-time distributions of juvenile 
and adult euphausiids off the coast of California. 

üllESTION 4.4 What is the general panero of vertical migration displayed 
by euphausiids in Figure 4.15. and hovv do the migrations of adults and 
juveniles difler? 

Each species has its own prefeired day and night depth range, and this may 
vary with the life stage (as illustrated in Figure 4.16) or with sex of an 
individual (e.g. adult female Calanus finmarchicus are strong migrators, 
whereas males are not). The preferred depth range may also change with 
season, geographic location, and general weather conditions (e.g. cloudiness, 
storm turbulence, etc.). In general, however, there are three patterns shown 
by migrating marine zooplankton: 

1 Nocturnal migration is characterized by a single daily ascent, usually 
beginning near sunset, and a single descent from the upper layers which 
occurs near sunrise. This is the most common pattern displayed by marine 

2 Twilight migration is marked by two ascents and two descents every 
24 hours. There is a sunset rise to a mínimum night-time depth, but during 
the night there is a descent called the midnight sink. At sunrise, the animáis 
again rise toward the surface, then later descend to the daytime depth. 

3 Reverse migration is the least common pattern. It is characterized by a 
surface rise during the day and a night-time descent to a máximum depth. 

The vertical distance travelled over 24 hours varíes, generally being greater 
among larger species and better swimmers. But even small copepods and 
small thecosomes may migrate several hundred metres twice in a 24-hour 
period, and stronger swimmers like euphausiids and pelagic shrimp may 
travel 800 m or more. Upward swimming speeds of copepods and the larvae 
of barnacles and crabs have been measured at 10-170 m h _1 , and 
euphausiids swim at rates of 100-200 m h _1 . Although the depth range of 
migration may be inhibíted by the presence of a thermocline or pyenoeline, 
this is not necessarily so, and an animal may traverse strong temperature and 
density gradients, as well as considerable pressure changes, during its 

Diel vertical migrations are responsible for the production of moving deep 
scattering layers (or DSLs). These are sound-reflecting layers picked up by 
sonar traces. They look like false sea bottoms on echograms (Figure 4.17), 
and they were initially believed to be the result of physical phenomena. 
However, these layers may move over 24 hours, and their rhythm provided a 
clue that they were caused by the movements of animáis. During the day, as 
many as ñve scattering layers may be recorded at depths of about 
100-750 m. At night, the layers rise almost to the surface and diffuse, or 
they merge into a broad band extending down to about 150 m. These deep 
scattering layers seem to be most frequently caused by the movements of 
larger crustaceans (e.g. euphausiids, shrimp) and small fish that possess 
sound-reflecting air bladders (e.g. myctophids), but other zooplankton such 
as heteropods and large copepods can occasionally form sound-reflecting 
layers too. 


i-1 i i-1-1-1-1-1-1- 1 -1-p-1-1-1—“i-1-1-1-r 



üsiKi ; r 

Figure 4.17 An echogram showing day-time 
deep scattering layers produced by euphausiids 
(ca. 90-150 m), fish (ca. 75-100 m) and 
unidentified animáis {ca. 175 m) in Saanich 
Inlet, British Columbia, Cañada. Note that the 
fish show up as discrete dots, whereas the 
smaller but more abundant euphausiids produce 
a more even shading pattern. Note also the 
irregularity of the seafloor, with an abrupt rise 
from about 225 m to 100 m in the centre of 
this transect. 

The fact that diel vertical migrations are tuned to the natural light.dark cycle 
suggests that changes in ambient light intensity may be of primary 
importance as stimuli in initiating and timing the migrations. Light intensity 
changes can also act as orienting cues for vertically migrating animáis, as 
can gravity and changes in hydrostatic pressure. Natural changes in light 
intensity which occur seasonally or even daily (e.g. sunny vs. cloudy days; 
dark nights vs. moonlit nights) can alter the depth ranges inhabited by 
particular species. Under continuous light in the Arctic summer, migrations 
may be totally suppressed. Solar eclipses will cause animáis to begin an 
upward migration during the day as light intensity decreases. In the 
laboratory, the timing of migrations may or may not change to conform with 
experimental alternations of light and dark periods. Factors other than light 
may also play a role in initiating the diel migrations; among those suggested 
as a causal mechanism is hunger, driving animáis upward toward the more 
productive areas under the protective cover of darkness. 

While light or other factors may trigger the diel vertical migrations of 
pelagic organisms, it does not expía in why so many species should show 
this behaviour. Whal is the adaptíve valué of vertical migration? 

Many hypotheses have been advanced to answer this question, but it may 
not be realistic to insist on a universal mechanism governing diel vertical 
migration in all species. It is important to recognize that the hypotheses 
discussed below may not be mutually exclusive, and that each may be more 
applicable to some species than others. 

1 One hypothesis is that animáis which remain in darkness or 
near-darkness over 24 hours are less vulnerable to visual predators, and this 


can be achieved by a daylight descent. The upward migration returns the 
animáis to the surface where food is most abundant. 

Considerable evidence supports this hypothesis as being the ultimate cause 
of DVM in both marine and freshwater species. It has been shown, for 
example, that diel migrations in several zooplankton species may become 
more pronounced when predatory fish are more abundant. As well, a 
45-year-long study of Metridia lucens in the North Atlantic has shown that 
the length of time this copepod was present near the surface varied 
seasonally, being shorter in the summer when nights are shorter. However, 
when food was most abundant during the spring, the animáis remained 
longer at the surface than was predicted from length of daylight; the 
importance of obtaining food when it is most abundant seems to override the 
importance of predator avoidance at this time. 

2 A second idea is that zooplankton conserve energy by spending 
non-feeding time in deeper, colder water where metabolic energy demands 
are less. It has not been proved that the energy saved in a colder 
environment would offset the amount of energy used in swimming during 
migration. However, energy required for swimming is very low, generally 
only a few percent of basic metabolic energy. 

3 A third hypothesis recognizes that zooplankton moving vertically in the 
water column are subjected to currents moving in different directions at 
different speeds. Thus they encounter a new feeding area each time they 
ascend. The new feeding area may contain more or less food than the area 
occupied the night before but, by migrating vertically, small organisms of 
limited mobility can avoid remaining in an area of little food as well as 
overgrazing any very productive area. However, experimental manipulations 
of food concentrations produce conflicting results depending on the species. 
In some cases, low food levels suppress vertical migration; in other 
examples, the reverse is true. 

Diel vertical migration has several consequences that are biologically and 
ecologically importante One is that, since all individuáis of a species do not 
migrate at precisely the same time and to the same depths, a population will 
eventually lose some individuáis and gain others. This mixture of individuáis 
from different populations enhances genetic mixing and is especially 
important in species of limited horizontal mobility. 

Another important result of vertical migration is that it increases and hastens 
the transfer of organic materials produced in the euphotic zone to deeper 
areas of the sea. The ladder-like series of migrating organisms (Figure 4.15) 
plays an important role in marine food chains. Each migrating animal 
removes food from shallower depths during the night; this material is then 
actively transported to deeper areas in the daytime. Herbivores remove 
phytoplankton from the euphotic zone, then migrate to deeper areas where 
they release faecal pellets and other organic debris and where they may be 
eaten by deeper-living camivores. The carnivores and scavengers in tum 
carry out vertical migrations at greater depths. The active vertical transpon 
of organic materials, either in the form of the animáis themselves or in their 
faecal pellets and other wastes is significantly faster than the passive sinking 
of organic particles. 



In some species, vertical migration patterns change seasonally and may be 
associated with breeding cycles and changing depth preferences of different 
stages in the life cycle. In the North Pacific Ocean, the dominant copepods 
show dramatic changes in their depth patterns. In inshore waters off the 
western coast of Cañada, Neocalanus plumchrus adults do not feed, and they 
overwinter at about 300-450 m depth where the eggs are laid between 
December and April (Figure 4.18). The eggs float toward the surface, and 
nauplii (see Section 4.2) hatch and develop at intermedíate depths. Nauplii 
are present in near-surface waters from February to April, and the populadon 
matures to the copepodite V stage during March to June when primary 
productivity is highest. By early June, stage V individuáis contain large 
amounts of lipids accumulated from feeding on phytoplankton, and they 
begin to migrate to deeper waters where they will subsist on this stored fat 
reserve. There they mature to the adult stage VI, mate, and lay eggs during 
the winter. In offshore waters, the life cycle changes somewhat, with 
spawning in deep water (>250 m) taking place from July to February and 
early copepodite stages first appearing in the upper 100 m in October. 
Nevertheless the species continúes to show a seasonal migration between 
surface waters, where larval development takes place, and deeper waters, 
where mating and spawning occur. A similar pattem of vertical migration 
associated with different reproductive stages takes place in Neocalanus 
cristatus , a large copepod also common to the North Pacific: adults are 
present between 500 m and 2000 m, and spawning occurs in deep water; 
younger stages move upward and live mostly above 250 m. 

Figure 4.19 shows both the seasonal and diel vertical migrations in two 
species of North Atlantic herbivorous copepods, Calañas helgolandicus and 
C. finmarchicus. During winter in the Celtic Sea, copepodites V and VI of 
both species are distributed fairly uniformly from the surface to about 100 m, 

Figure 4.18 A schematic diagram of the life cycle of the copepod Neocalanus plumchrus in 
Coastal waters off British Columbia, Cañada. The depth distributions of the eggs, larvae 
(nauplii l-VI and copepodites l-V) and adults (copepodite VI) are shown over the course of 
one year. C, copepodite: N, nauplius. 

depth (m) g depth (m) 


(a) Catanas helgolandicus 

January April July August September 

3í1 20 10 0 10 70 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 % 

temperatura (*C) 

Catanas finmarchicus 

Figure 4.19 Seasonal changes in the day-time (white) and night-time (black) vertical 
distribution of copepodite stages V and VI of two species of copepods, Calanus 
helgofandicus (a) and C. finmarchicus (b) T in the Celtic Sea. Numbers in each plankton haul 
are plotted in 5 m depth intervals as percentages of total numbers (n) present in the haul. 
Temperature profiles are shown for the day hauls and apply to both species. 

and there is little difference between day and night-time distributions. In 
spring (April), both species begin to concéntrate in shallower depths, and 
they display diel vertical migrations. In July and August, the thermocline 
becomes well established and the two species show a clear separation in their 
distributions. Calanus helgolandicus continúes to develop in the warmer 
surface zone and to display diel migration, but C. finmarchicus moves 
deeper into cooler water beneath the thermocline and shows little difference 
in day and night depth preferences. By late September, both species reside in 
water deeper than 40 m during the day, and C. helgolandicus maintains its 
strong vertical movement toward the surface at night. 

QUESTI0N 4.5 Of what advantage is the seasonal change in vertical 
migratory patterns to Calanus helgolandicus and C. finmarchicus! 

The Antarctic krill (Euphausia superba) also undergoes extensive depth 
changes during its life cycle. The eggs of the krill are deposited in surface 
waters but sink rapidly to depths of 500-2000 m, where they hatch. The 
larvae then gradually float and swim to the surface where development is 
completed, and juveniles and adults are found at or very near the surface. 


The total life span is estimated to be at least 2-4 years. During this time, the 
vertical migration of the different stages into currents moving in different 
directions results in transport away from, and back toward, the Antarctic 
continent. Several common Antarctic species of copepods and chaetognaths 
also undergo similar seasonal vertical migrations: in the Southern summer, 
the species are present in surface water flowing northward from the area 
known as the Antarctic Divergence; in the winter, they are found in a 
southerly-flowing deeper current which rises to the surface at the Divergence. 

Extensive seasonal migrations are generally undertaken only by species 
living in températe and coid waters, or in upwelling regions. The migrations 
usually result in young animáis being within the productive surface waters at 
a time when they can obtain sufficient quantities of food for growth. In 
températe waters, as production declines in the surface waters during summer 
and fall, late larval stages or sexually mature adults move to deeper waters. 
Here, in colder and unproductive waters, they may enter a State called 
diapause in which their metabolism slows and they do not feed. Instead, 
they subsist on energy reserves built up during their stay in the surface zone. 

Both diel and seasonal vertical migrations place migrants in currents that are 
moving in different directions and at variable speeds. Despite this, 
populations of marine zooplankton do persist in their own characteristic 
geographical regions. A distinctive pattem of seasonal vertical migrations in 
a species may ensure retention within an appropriate habitat (as it does with 
Antarctic zooplankton), or within a productive upwelling area (as is thought 
to be the case for several species associated with annual upwelling cycles off 
the western coasts of North America and Africa). Diel migrations may 
similarly retain animáis in favourable habitats. For example, the diel 
migrations of estuarine species may be attuned to the inward and outward 
tidal flows of water to retain the animáis within the estuary. In all such 
cases, natural selection favours those individuáis that migrate with 
appropriate timing; any individuáis with behavioural pattems that do not 
conform to the physical System will tend to be lost from the región. The 
corollary to this is that at least some species are known to exhibit different 
vertical migratory pattems in different locations, further suggesting that these 
pattems can be modified to ensure persistence of populations in particular 
geographic regions. What appears to be a confusing variability in migratory 
pattems among and within species may be the result of evolutionary 
adaptations leading to maintenance of populations in favourable 
environments of different current regimes. 


Zoogeographic studies describe the distributions of living organisms and 
investígate the physiological and ecological causes underlying the patterns. 
Such studies may also be historical, as the present-day distributions of 
marine organisms also have been determined by events and changes taking 
place over geological time. 

Compared with terrestrial environments, the pelagic realm has few physical 
barriers to impede mixing and gene flow between populations of 
holoplanktonic organisms. But there are hydrographic barriers between 
different water masses which have distinctive complements of 
physico-chemical conditions and ecological properties. Some zooplankton 


have wide distributions encompassing a broad spectrum of environmental 
determinants, but others are restricted to such narrow limits of temperatura, 
salinity, and other factors that they can be used as biological indicators of 
the particular water-mass types they inhabit. The concept of using indicator 
species to characterize specific water masses has most frequently been 
applied to certain species of foraminifera, copepods, and chaetognaths, as 
they are usually sufficiently abundant to be sampled routinely (see, for 
example, Figure 6.9). 

The sharp north-south temperature gradients set major environmental 
provinces in the ocean surface waters as described in Section 2.2.1. Even so, 
roughly 50% of all epipelagic zooplankton species extend from tropical and 
subtropical waters into at least part of the températe zone. Only about 
one-third of the epipelagic holoplankton are restricted to the warm waters of 
the tropics and subtropics. Fewer species are restricted to coid and/or 
températe waters, but included in this category are several species with 
bipolar distribution. Bipolar species are found in both arctic/subarctic water 
and antarctic/subantarctic regions, but are not present in intervening regions. 
They inelude the pteropods Limacina helicina and L. retroversa , the 
amphipod Parathemisto gaudichaudi , and a siphonophore ( Dimophyes 
árctica) as well as several diatoms. There are also Arctic and Antarctic 
species which are so closely similar that they certainly indicate a common 
ancestry. An example of these ‘species pairs’ would be the gymnosomes 
Clione limacina (present in the Northern Hemisphere) and C. antárctica 
(Southern Hemisphere); although morphologically distinguishable, these 
species occupy the same position in polar food chains where both feed 
exclusively on Limacina helicina and L retroversa. Bipolarity may have 
arisen from animáis being transported in the deep, coid water link between 
the north and south polar regions. An alternative theory proposes that 
formerly cosmopolitan species were displaced from lower latitudes by 
competition with other zooplankton, leaving relict populations in the high 

There are fewer latitudinal hydrographic barriers in deeper water (see 
Section 2.4), and meso* and bathypelagic plankton generally have relatively 
wide distributions. However, they are by no means cosmopolitan; that is, 
many species are restricted to one of the major oceans. For example, it is 
believed that approximately half of the bathypelagic fauna of the North 
Pacific is endemic to that area. About 20% of the deep-living North Pacific 
copepods are Antarctic forms, which is not unexpected because the Antarctic 
is the source of much of the deep water in all the oceans (see Section 2.4 
and Figure 2.17). 

The number of epipelagic species decreases from low to high latitudes, a 
phenomenon seen also in terrestrial animáis. Most of the different 
zooplankton groups have fewer species in coid waters, and some (e.g. 
heteropods) have no representatives outside of subtropical boundaries (about 
45°N-45°S). As species diversity declines in high latitudes, there is a reverse 
tendeney for cold-water epipelagic species to have greater numbers of 
individuáis. Although many hypotheses have been proposed to explain these 
latitudinal differences in diversity and abundance, the reasons 
remain debated. 

There are a relatively large number of pelagic marine species that are 
described as ‘circumglobal tropical-subtropicaP. These species are present in 
warm waters of the Atlantic, Pacific and Indian Oceans. They inelude many 


of the neustonic animáis such as Janthina species and Glaucus atlanticus 
(see Section 4.4), as well as representatives of epipelagic euphausiids, 
chaetognaths, amphipods, and other major groups. This wide present-day 
distribution can be attributed to the long continuity of the warm water 
masses of the world via the ancient Tethys Ocean that existed for several 
hundred million years, from the Paleozoic until the Late Tertiary (see 
Appendix 1 for dates). Many warm-water species of zooplankton, however, 
are restricted to one or two of the main oceans, and their distributions may 
be related to present-day barriers (e.g. the Isthmus of Panama) that now 
cióse former routes between major oceans. 

Do humaos influence the distributions of zooplankton? 

Constructions such as the Suez Canal have affected zooplankton 
distributions. Since this canal was opened in 1869, about 140 species have 
entered the Mediterranean from the Red Sea. Zooplankton distributions also 
may be changed by accidental transpon in the ballast water of ships (see 
also Section 9.3). This is believed to be the way in which the ctenophore 
Mnemiopsis leidyi moved from the Atlantic coast of North America into the 
highly polluted Black Sea. Within two to three years, this predatory species 
increased to a biomass of about 10 9 tonnes in 1990. Its effect on biological 
communities and fish stocks in the Black Sea is estimated to be greater than 
that of all other anthropogenic factors. 

The North Atlantic and North Pacific Oceans have been separated by the 
land mass of North America for approximately 150-200 million years. Yet 
the températe zones of these oceans show many similarities in their resident 
fauna (Table 4.2). The medusa Aglantha digitale , the chaetognath Eukrohnia 

Table 4.2 Numerically dominant net-collected zooplankton species in the epipelagic 
zones of the northem North Atlantic and northem North Pacific. Species aligned 
between the two columns are found in both oceans. 


North Atlantic Ocean 

North Pacific Ocean 


Aglantha digitale 


Tomopteris septentrionalis 


Eukrohnia hamata 

Sagitta serratodentata 

Sagitta maxima 

Sagitta elegans 


Parathemisto pacifica 


Calanus finmarchicus 

Calanus pacificas 

Calanus helgolandicus 

Neocalanus plumchrus 

Euchaeta norvegica 

Neocalanus cristatus 

Pleuromamma robusta 

Eucalanus bungii 

Acartia clausi 

Acartia longiremis 

Metridia lucens 

Metridia pacifica 

Oithona spp. 

Oithona similis 

Oncaea spp. 

Scolecithricella minor 

Heterorhabdus norvegicus 

Pseudocalanus minutus 
Paracalanus parvus 


Meganyctiphanes norvegica 

Euphausia pacifica 

Thysanoessa longicaudata 

Thysanoessa longipes 


Limacina helicina 

Limacina re trove rsa 

Clione limacina 


Salpa fusiformis 


hamata , the pteropods Limacina helicina and Clione limacina are all 
examples of species found in both areas, as well as in the Arctic Ocean. On 
the other hand, the dominant copepods and euphausiids of the North Pacific 
and North Atlantic are mostly of different species, although some are in the 
same genera (Table 4.2). Speciation and present-day distributions in these 
areas have been influenced by geological changes marked by intermittent 
changes in water flow between the Pacific, Arctic, and Atlantic oceans. At 
the present time, there is almost no southward flow of water through the 
Bering Strait, but there is a flow from the Pacific into the Arctic as there 
presumably has been whenever the Strait has been open. There is a much 
greater water exchange between the Arctic and the Atlantic. Present-day 
distributions suggest that the Arctic was the passageway between the 
Atlantic and Pacific for dispersal of many planktonic species. However, 
cooling of the Arctic during the Pliocene and Pleistocene periods (see 
Appendix 1) may have resulted in the many examples of species with 
discontinuous ranges (found in the northern Atlantic and Pacific but not in 
the Arctic) and in subsequent evolution into distinct species. 

An unbroken circumglobal ocean lies in the Southern Hemisphere between 
Antárctica and the continents of Africa, Australia, and South America. 
Zooplankton in this broad oceanic area exhibit continuous, concentric 
patterns of distribution around the Antarctic continent which conform to 
concentric isotherms and the general clockwise circulation pattern. However, 
there are also currents that flow northward from the southward toward 
Antárctica and, as pointed out in Section 4.6, seasonal vertical migrations in 
these currents also serve to maintain populations around the continent. The 
pelagic fauna of the Antarctic Ocean is much richer in numbers of species 
than that of the Arctic Ocean, and this difference may be related to the 
higher productivity of the Southern ocean. 


Within the geographical boundaries inhabited by any species, the individuáis 
of that species are not distributed uniformly or randomly, but are usually 
aggregated into ‘patches’ of variable size. This patchiness is true of both 
phytoplankton and zooplankton, as well as of other types of marine and 
terrestrial species. Patchiness in phytoplankton distribution was introduced in 
Section 3.5, where it was related to physical processes that control nutrient 
availability and thus plant production on scales ranging from oceanic gyres 
to Langmuir cells of circulation. Zooplankton patchiness may be correlated 
with phytoplankton concentrations, or it may be caused by other factors. 

Small-scale heterogeneity in the horizontal distribution of zooplankton (also 
known as microdistribution) is more difficult to detect and study than broad 
geographic patterns because of the way zooplankton are collected. Nets are 
generally towed through the water for distances ranging from tens of metres 
to kilometres, so that the numbers of collected individuáis are averaged over 
distances that will mask any smaller-scale patterns. Specially designed 
sampling programs have demonstrated microdistributional patterns in 
zooplankton, as have direct observations by scuba divers and observers in 
submersibles. Patchy distribution on these smaller scales can be explained in 
a number of ways, and may be related to physical, Chemical, or 
biological events, 

Various types of horizontal and turbulent mixing can result in aggregation 
or dispersión of planktonic populations. As discussed in Section 3.5, some 


types of mixing (upwelling) result in elevated surface nutrient concentrations, 
high primary production, and increased numbers of zooplankton; other forms 
of mixing (downwelling) have the opposite effect on production and 
aggregation of organisms. Zones of vertical mixing range in area from very 
large shelf-break fronts (Section 3.5.4), to smaller scale coid- or warm-core 
rings (Section 3.5.1), to much smaller Langmuir circulation pattems 
(Section 3.5.6), all of which can affect zooplankton distribution and 
abundance on corresponding spatial scales. Differences in scale are shown in 
Figure 4.20, which illustrates changes in numbers of zooplankton (and 
phytoplankton) on a scale of kilometres, and in Figure 4.21, which shows 
smaller-scale patchiness of zooplankton on a scale of metres. These figures 
¿Ilústrate patchiness on a horizontal axis, but zooplankton also form discrete 
aggregates in the vertical dimensión. Figure 4.22 shows the vertical 
distribution of copepods in the Bering Sea; note the vertical separation of the 
species within the epipelagic and mesopelagic zones, as well as the discrete 
depths inhabited by different life stages and by males and females. 

What are some of the biológica! or ecológica! causes of patchy distribution 
of zooplankton? 

Patchiness may result from interactions between zooplankton and their food. 
On time scales of months, high primary production may result in high 
secondary production (as in Coastal upwelling) but, on shorter time scales 
and in smaller areas, dense aggregations of phytoplankton and of 
herbivorous zooplankton tend to be mutually exclusive (Figure 4.20). This 
may result from heavy grazing by the herbivores which reduces the numbers 
of phytoplankton. It also may be the result of differences in growth rates 
between the algae and the zooplankton; whereas phytoplankton can quickly 
multiply under favourable light and nutrient concentrations, increases in 
numbers of zooplankton often lag considerably behind because of their 
slower generation times. Consequently, when phytoplankton numbers are 
peaking and nutrients are declining, zooplankton biomass may still be low as 
the animáis begin growing in response to the elevated food supply. 

Reproduction may also play a role in causing patchy distribution in some 
species. Aggregations of zooplankton formed for purposes of breeding will 

Figure 4.20 Patchiness in phytoplankton (as 
indicated by chlorophyll a concentraron) and in 
zooplankton, on a kilometre scale. Based on 
night-time data taken from 3 m depth in the 
northern North Sea, May, 1976. 


distance (m) 

Figure 4.21 Examples of small-scale (in metres) patchy distribution of zooplankton off the 
California coast. (a) A shelled pteropod. Limacina ; (b) a chaetognaíh, Sagitta ; (c) a copepod, 
Corycaeus ; and (d) euphausiid larvae. 

cause a small-scale heterogeneous distribution, although the mechanisms in 
which the members of the swarm unite are not understood. Also, all the 
progeny hatching from one swarm, or even from one egg mass, tend to 
remain together for some period of time before they become dispersed. 

Considerable attention has been given to the patchy distribution of Antarctic 
krill because of their abundance and consequent importance for higher 
trophic levels, and because of the potential for commercial harvesting of this 
species (see Section 6.1). When feeding, Euphausia superba forms swarms in 
which the individuáis are closely packed but move independently of each 

depth (m) 


relativa frequency {%) 

Eucaianus bungü + 

E . bungü CV 
/Wefr/d/a pacifica CV 

M. pacifica ? 

A/e o c a/an us cr/sfaíus CV 
A/, p/umc/íros CV 

N. plumchrus O 
N. píumchrus ? 
Pseudocalanus minutus ¥ 
P. minutus copepodites 
Scoiecithriceiia minar ¥ 

Figure 4.22 Vertical zonation ot the copepod community in the Bering Sea during summer. 
Samples were collected during daylight hours. ?, females;</, males; CV, copepodite stage V 
(see Section 4.2). 

other. At other times, the euphausiids are organized in schools, in which the 
individuáis are uniformly oriented and swim together at a uniform speed. 

The formation of schools is thought to offer some defence against certain 
predators but, on the other hand, some predators may forcé schooling in 
order to concéntrate their prey. For example, some températe-water species 
of euphausiids may be driven into tight schools by sharks, or by whales 
which produce a ‘net’ of bubbles to encircle and concéntrate their prey. In 
general, extremely high concentrations of predators (e.g. swarms of medusae 
or ctenophores, or fish schools) will quickly cause local decreases in the 
numbers of their prey and thus create a patchy distribution of the prey. 

Vertical patches of individuáis and species, as shown in Figure 4.22, will 
change over 24 hours as some animáis migrate vertically (see Section 4.5). 
In general, migrators tend to be more dispersed during the night, and to form 


denser aggregations during the day in deeper water. Vertical separation may 
be due to physical factors that inelude the presence of pyenoelines and 
thermoclines, to light intensity preferences, or to other microenvironmental 
differences. Vertical aggregations also may result from the distribution of 
preferred food Ítems, from predation, or from other biological factors. 

Table 4.3 summarizes some of the physical and biological processes that 
cause the patchy distribution of planktonic organisms. As mentioned above, 
spatial scales vary from thousands of kilometres to very small-scale patches 
of 10 m or less. The length of time a particular patch of plankton may 
persist varies according to the cause of the distribution. Very large patches of 
zooplankton, such as those caught in rings spun off from the Gulf Stream, 
may persist for months or even years. Mating aggregations of 
macrozooplankton (e.g. euphausiids) or of nekton (squid, fish) may persist 
for only a few days, but the planktonic offspring that hatch from spawning 
aggregations may remain together for many days or months because they 
will be less independent of water movements than the adults. Patchiness due 
to Langmuir circulation will persist only as long as wind velocity and 
direction remain constant; and wave action may cause constantly changing 
patterns of aggregation and dispersal in near-surface plankton. Thus there is a 
continuum in size of patches from very large to very small horizontal scales, 
and in persistence of patches from thousands of days to momentary periods. 

Table 4.3 Approximate spatial and temporal scales of some important processes 
that cause patchy distribution of zooplankton. 

length scale 





time scale 


Gyres (e.g. Sargasso 
Sea); continental 
upwelling (e.g. Perú 
Current); water 
mass boundaries 
(e.g. Antarctic 

Regional ecosystems 
defined by the water 



Warm and coid 
core rings; tidal 
fronts; seasonal 

Coastal upwelling 

Seasonal growth (e.g. 
spring blooms); differ- 
ential growth between 
phyto-and zooplankton 

Lunar eyeles (e.g. fish 



Turbulence (e.g. 
estuarine mixing; 
island wake effeets) 

Reproductive eyeles 




Tidal mixing 

Diel events (e.g. vertical 



vertical mixing 

Physiological adapta- 
tion (e.g. buoyaney; 
light adaptation) 



Langmuir circulation; 
wave action 

Behavioural adaptation 
(feeding swarms) 



QUESTION 4.6 Referring co Figure 4JO* (a) can you provide any explana!ion 
íbr why the amount of ehloropfoyll a general ly inversely correlated with 
zooplankton numbers at any locality? ib) Would you expect the numbers of 
zooplanklon at these localities and depth to increase or decrease during 
daylight hours? 


Records showing long-term changes in plankton community structure are 
available for only a few marine areas, but they indícate that there can be 
considerable variation in the abundance and species composition of 
zooplankton on decadal time scales. Often these changes in plankton 
communities are correlated with changes in atomospheric and marine 
climate. Long-term climate changes that significantly alter marine 
ecosystems and biological production are known as regime shifts, and 
several examples are given below. 

Some of the longest zooplankton records come from the north-east Atlantic 
Ocean, where continuous plankton recorders (CPRs) have been towed by 
commercial ships on regular routes for almost 50 years. Figure 4.23 shows a 
general decline in both phytoplankton and zooplankton abundance in this 
región over the past 40 years, except for small increases during the early 
1980s. A similar pattem has been shown off Southern California, where the 
macrozooplankton biomass was 70% lower in 1987-93 than in 1951-57. In 
this Coastal area, ocean climate changes were correlated with the decrease in 
plankton. During the 40 years of declining plankton biomass, the sea surface 

Figure 4.23 Long-term fluctuations in the abundance of phytoplankton and zooplankton in 
the north-east Atlantic Ocean. The blue solid line represents standard deviation units from 
the long-term annual mean; the blue broken line has been statistically smoothed to show the 
average trend. 


temperature off California increased by about 1.5 C°, and the temperature 
difference across the thermocline increased. The increased stratification of 
the water column lessened wind-driven upwelling, and consequently the 
lower-nutrient regime depressed phytoplankton production and supported 
fewer zooplankton. It is not clear whether these changes are part of natural 
climatic cycles that will reverse in coming years, or whether the zooplankton 
decline is due to global warming. If temperatures continué to increase 
globally and stratification increases throughout the oceans, the biological 
impacts could be drastic in terms of lowering marine production in areas 
where it is presently enhanced by upwelling of nutrients. 

Can changes in ocean di mate increase biological production? 

At about the same time that plankton biomass was declining in the 
California Current, plankton abundance was increasing in other parts of the 
Pacific. In the central North Pacific Ocean (ca. 26°-3I o N to 150°-158° W), 
total chlorophyll a in the water column doubled from 3.3 mg m~ 2 during 
1968-1973 to 6.5 mg m -2 in 1980-85. Farther north, in the subarctic 
Pacific Ocean, there was a doubling of zooplankton biomass and a similar 
increase in pelagic fish and squid abundance between the periods 1956-1962 
and 1980-89. In both of these areas, the enhanced production has been 
correlated with an increase in the intensity of winter winds, which increase 
vertical mixing and bring more nutrients into the euphotic zone. 

There may also be long-term changes in species composition of plankton 
communities. In the central North Sea, holoplanktonic calanoid copepods 
(see Section 4.2) dominated the mesozooplankton community from 1958 to 
the late 1970s. In the 1980s and early 1990s, meroplanktonic larvae of sea 
urchins and brittle stars (see Sections 4.3 and 7.2.1) became numerically 
dominant and more abundant than any single holoplanktonic species. This 
change in zooplankton composition was reflected in a 2- to 8-fold increase in 
the abundance of the macrobenthos of the area. The reason for a preferential 
increase in benthic echinoderms in the North Sea remains unclear. 


1 The marine zooplankton community ineludes many different species of 
animáis, ranging in size from microscopic protozoans to animáis of several 
metres in dimensión. The holoplanktonic species spend their entire lives in 
the pelagic environment; meroplanktonic forms are temporary members of 
the plankton, and inelude the eggs and larval stages of many benthic 
invertebrates and fish. 

2 Although zooplankton are routinely collected by towing fine-meshed nets 
through the water, not all species are representatively captured by this 
method. Some animáis are too small to be retained in nets, others are 
capable of detecting and evading nets, and some species are too fragüe to 
survive collection by nets and subsequent processing in Chemical 
preservad ves. Direct observations of zooplankton using scuba techniques, 
ROVs, or submersibles have greatly increased our knowledge of fragüe 
and/or fast-swimming species. 

3 The presence of meroplanktonic larvae in the water is linked to the 
reproductive pattems of the adults. In tropical regions, meroplankton are 
present throughout the year. In higher latitudes, the larvae of benthic 


invertebrates and fish appear seasonally because reproduction in the adults is 
linked to higher temperatures and elevated phytoplankton production. 

4 The vertical gradients of temperature, light, primary production, pressure, 
and salinity create distinctive environments at different depths in the water 
column. These vertical zones (epi-, meso-, bathy-, and abyssopelagic) are 
somewhat arbitrary in nature, but different species of zooplankton generally 
inhabit discrete depth zones within the ocean. The life styles, morphology, 
and behaviour of organisms living deeper in the water column differ from 
those exhibited by epipelagic species, and the biomass of zooplankton 
decreases exponentially with depth. 

5 As light from the Sun diminishes with depth, bioluminescent light 
produced by organisms becomes increasingly important as a means of 
communication. Many different species display the ability to produce light, 
and the biological significance of bioluminescence varíes with the species. 
Some use light displays to attract potential prey, others to deter predators; 
some may use bioluminescence to attract mates, or to form 
reproductive swarms. 

6 Although most zooplankton have preferred depth ranges, many species 
move vertically in the water column with a diel periodicity. The most usual 
pattem is a nocturnal migration in which animáis make a single ascent 
toward the surface at night, followed by a single descent to deeper water at 
sunrise. The adaptive significance of diel vertical migration may be different 
for different species. This behaviour may allow animáis to conserve energy 
by remaining in colder waters except when feeding; it may reduce mortality 
from visual predators; or it may permit animáis of limited swimming ability 
to sample new feeding areas with each ascent. 

7 Diel vertical migration has several important biological and ecological 
consequences. It probably enhances genetic exchange by mixing the 
members of a given population; this results because vertical migrations are 
never precisely synchronized among all the members of a population. Some 
individuáis begin migrations sooner or later than others, with the result that 
some members will eventually be lost from the original group and new 
members will be added. Secondly, diel vertical migrations increase the speed 
at which organic materials produced in the euphotic zone are transferred to 
deeper areas. 

8 In high latitudes, extensive vertical migrations may be undertaken on a 
seasonal basis, and these are generally linked with reproductive cycles and 
development of larval stages. In such migrations, the adults are usually 
found in deeper waters during the winter when food is scarce; the 
developing young are present in surface waters during the spring and 
summer when phytoplankton is plentiful. 

9 By moving vertically in the water column, zooplankton enter currents 
that are moving in different directions and at different speeds. Thus diel or 
seasonal vertical migrations that are attuned to particular current regimes can 
result in the retention of populations within favourable localities. 

10 Present-day distributions of zooplankton have been established over 
geological time and reflect past dispersal patterns as well as the 
physiological and ecological requirements of the species. 

11 Epipelagic zooplankton are often associated with specific water mass 
types, which are established by latitudinal gradients in temperature, salinity, 


and other physico-chemical factors. Mesopelagic and bathypelagic species 
tend to have wider geographic distributions, reflecting increasing 
homogeneity in environmental conditions with increasing depth. 

12 The numbers of species of epipelagic and mesopelagic zooplankton are 
higher in low latitudes, but the numbers of individuáis tend to be relatively 
low. The reverse situation is found in high latitudes, where there are fewer 
species but with higher abundance. 

13 Within the bounds of their geographic regions, zooplankton exhibit 
patchy distributions on a wide range of space- and time-scales. Patchiness 
may result from responses to physical turbulence or mixing, or to Chemical 
gradients such as salinity changes. Patchiness may also result from 
interactions between prey and predators, or it may reflect other biological 
events such as reproduction. 

14 Long-term records indicate that plankton abundance and species 
composition may change substantially over decadal time scales. Decreasing 
plankton biomass may be caused by climate changes that increase water 
stratification and depress upwelling; conversely, in other regions, increasing 
winds may enhance nutrient concentrations in the euphotic zone and lead to 
increased phytoplankton and zooplankton production. 

Now try thefollowing questions to consolídate your understanding ofthis 

QUESTION 4.7 Why do the planktonic Crustácea tend to have so munv 
difieren! growth si ages? 

QUESTION 4.8 Why do so many benthic species of animáis produce 
meroplankton ic larvae? 

QUESTION 4.9 Which planktonic organisms produce skeletal maten ais that 
contribute to sedimenta on the seaRoor? 

QUESTION 4.10 Oí ihe major zooplankton groups listed in Table 4.2. which 
are predominuntly carnivorous and which are predominantly herbivorous? 
(Refer to Section 4.2.) 

QUESTION 4.11 Whai is the advantage of patchy distribution of plankion 
predators that actively seek out their food? 

QUESTION 4.12 In Figure 4.22. only the copepodite V stage of Neocakmus 
Cristinas is shown, Where and when would you expect to find adults of ibis 
species? (Refer lo Section 4.6.) 





Food chains are linear arrangements showing the transfer of energy and 
organic materials through various trophic levels of marine organisms. Each 
trophic level is composed of organisms that obtain their energy in a similar 
manner. The pelagic food chain begins with the phytoplankton; these 
autotrophic primary producers, which build organic materials from 
inorganic elements, form the first trophic level. Herbivorous species of 
zooplankton that feed directly on the marine algae (e.g. Protozoa, many 
copepods, salps, larvaceans) make up the second trophic level, and they are 
referred to as primary consumers. Subsequent trophic levels are formed by 
the carnivorous species of zooplankton that feed on herbivorous species 
(secondary consumers like chaetognaths), and by the carnivores that feed 
on smaller carnivores (tertiary consumers including many jellyfish and 
fish). The total number of trophic levels will vary with locality and with the 
total number of species in the community. The highest trophic level is 
occupied by those adult animáis that have no predators of their own other 
than humans; these top level predators may inelude sharks, fish, squid, and 
mammals. The total amount of animal biomass produced in all higher trophic 
levels, per unit area and per unit time, is called secondary production (as 
opposed to the primary production of plants). Trophodynamic studies 
examine the factors that affect transfers of energy and materials between 
trophic levels and that ultimately control secondary production. 

Elements such as nitrogen, carbón and phosphorus, which become 
incorporated in organic components of plant and animal tissues, have a 
eyelieal flow through food chains (Figure 5.1). Bacteria decompose waste 
materials and the tissues of dead organisms. Decomposition releases 
inorganic forms of essential elements, and these become available again for 
uptake by autotrophic organisms. Energy, however, has a unidirectional flow 
(Figure 5.1). Some energy is lost at each transfer to the next trophic level 

Figure 5.1 A schematic representation of 
mineral reeyeling and energy flow in marine 



because much of the Chemical energy incorporated in organic compounds is 
converted to heat energy and is dissipated in respiration, when organic 
carbón is broken down to CO 2 . As a consequence, the total energy will 
diminish at each trophic level, and this places a finite limit on the possible 
number of trophic levels in any community. 

question 5.1 Whai are the cunsequences of energy loss due to respiration on 
(a) the rehit i ve total numbers of organisms in suecessive trophic levels* and 
on (b) the relative amounts of primary produciiori and secondary production? 

The size of individual organisms generally increases within each succeeding 
trophic level, but the generation time (or length of the life cycle) becomes 
progressively longer. The generation times of phytoplankton are measured in 
hours or days, those of zooplankton in weeks to months; those of fish in 
years; and those of marine mammals in many years. One might expect to 
observe considerable differences between the standing stocks of 
phytoplankton and fish or whales, but it is believed that because of the 
dissimilarities in generation times, the total biomass in each succeeding 
trophic level decreases only very slightly (Figure 5.2). The biomass of the 
exceedingly numerous, microscopic, and rapidly reproducing phytoplankton 
is probably never more than four times that of the small numbers of very 
large marine mammals which have long generation times. 

We have seen that it is relatively easy to estimate primary production by 
marine phytoplankton (see Section 3.2.1). Because of the longer generation 
times of zooplankton and fish, and because it is difficult or impossible to 
follow populations of these animáis in the sea for any length of time, it is 
much harder to obtain estimates of pelagic secondary production. There are 
some methods that can be applied to data collected in the field, and these are 
discussed in Section 5.3.1, along with their limitations. Secondary 
production can also be studied by growing marine zooplankton and fish 
under experimental conditions, and this approach is considered in 
Section 5.3.2. Another approach is to use estimates of primary production 

Figure 5.2 The average biomass of organisms of different sizes in marine food chains. To 
elimínate differences in shape, the size of each type of organism has been converted to the 
diameter of a spherical particle having the same biomass as the organism. The upper line 
illustrates the average biomass of different organisms in the Antarctic Ocean, a región of 
high productivity. The lower line shows the biomass of typical organisms in the equatorial 
Pacific, an oceanic región of lower productivity. 


and our knowledge of marine food chains to predict secondary production 
and yields of fish. Using this indirect approach, it is necessary to know how 
much energy is transferred between each trophic level. 

The efficiency with which energy is transferred between levels is called the 
ecological efficiency (£), and it is defined as the amount of energy extracted 
from a given trophic level divided by the energy supplied to that trophic 
level Ecological efficiency is difficult to measure, and it can more easily be 
approximated from the transfer efficiency (£>)> defined as: 

Et = ~ (5.1) 


where P t is the annual production at trophic level /, and /Vi is the annual 
production in the preceding trophic level í — 1. In this equation, production 
can be defined either in terms of energy (e.g. measured conventionally in 
calories or, in the SI system, in joules) or in terms of biomass (e.g. carbón in 
grams). For the energy transfer between phytoplankton and zooplankton, Ej 
will equal the amount of herbivore production divided by the primary 
production. At the next step, the transfer efficiency will be the annual 
production of secondary consumers (i.e. camivorous zooplankton) divided by 
the annual production of herbivores. 

QUESTION 5,2 Calcúlate the transfer efficiency between phytopEankion and 
herbivorous zooplankton in a marine community where the net primary 
productivity is 150 g C ni 2 yr~ 1 and the annual production of herbivorous 
copepods is 25 g C m" 2 . 

In marine ecosystems, valúes for transfer efficiencies have been estimated at 
about 20% for the transfer from plants to herbivores, and at 15-10% at 
higher levels. This means that there are corresponding energy losses between 
trophic levels of 80-90%, primarily through respiration (the heat losses 
shown in Figure 5.1). 

QUESTION 5.3 Why would the respiral ion losses be § reaten and transfer 
efficiencies lower, when considermg higher trophic levels? 

Note that equation 5.1 deais with the consumption of energy in succeeding 
trophic levels. A certain amount of production is not consumed directly. Not 
all living phytoplankton and zooplankton are eaten; some die naturally, and 
the energy contained in dead organic material becomes available for 
scavengers or microbial decomposers in a different pathway (see 
Section 5.2.1). This detritus may be cycled either in the water column or in 
the benthic community. 

QUESTION 5.4 If a large amount of ihc primary production ¡s not eaten by the 
herbivorous zooplankton. hut dies and sinks out of the water column, what 
happens to the valué of the transfer efficiency between the se iwo trophic 

In addition to knowing how much energy is transferred between trophic 
levels, it is also necessary to know how many trophic levels there are in any 
particular locality for which secondary production will be estimated. Because 
of the energy losses incurred with each transfer between trophic levels, the 
number of links will partly determine the biomass of top-level predators (i.e. 


fish, squid, or marine mammals). There is reasonable evidence to suggest that 
the number of links in the pelagic food chain varíes with locality and may 
be determined by the individual size of the primary producers. The number 
of trophic levels vanes from up to six in the open ocean, to about four over 
continental shelves, to only three in upwelling zones, as shown in Figure 5.3. 
Note that when the size of the dominant phytoplankton is small, the food 
chain is lengthened, as in open ocean areas. In such situations, marine 
protozoans (zooflagellates and ciliates) become important intermediary links; 
they may consume a major fraction of the primary production, and in tum 
they constitute an abundant dietary source for suspension-feeding copepods 
or other zooplankton that are incapable of feeding directly on very small 

Figure 5.3 A comparison of food chains in three different marine habitats. The organisms 
representing each trophic level are only selected examples of the many marine species that 
could be present in that level. (Organisms not to scale.) 

I. Open ocean (6 trophic levels) 

nanoplankton microzooplanktonmacrozooplankton megazooplankton zoúpíanktivorous fish piscivorous fish 
(flagehates) (protozoa) ** (copepods) ^ (chaetognaths) ** (myctophíds) —(tuna, squid) 

IL Continental shelves (4 trophic levels) 

^ y v 

(diatoms, dinoflagellates) ; 


'{clams, mussels) - 

III. Upwelling regtons (3 trophic levels) 

planktivorous fish 




planktivorous whales 
(baleen whales) 


Table 5.1 The relation between recent estimates of primary production and fish 
production in three different marine habitats. 









ocean area 

Mean primary 




(g C rrr 2 yr" 1 ) 

Total plant 
(10 9 tonnes C yr -1 ) 




Number of energy 
transfers between 




trophic levels 







Mean fish 



44 700 


(mg C m" 2 yr -1 ) 

Total fish 





(10 6 tonnes C yr -1 ) 

*The number of trophic levels in upwelling areas may be 2 (if fish are predominantly 
herbivores) or 3 as represented in Figure 5.3; 1.5 represents an average valué for the 
number of energy transfers {n — 1 or 2). 

**Calculated from equation 5.2. 

***Corrected for percent ocean area occupied by each habitat (total 
area — 362 x 10 6 km 2 ). 

phytoplankton. In contrast, large chain-forming diatoms domínate in 
nutrient-rich upwelling regions, and short food chains result because large 
zooplankton or fish can feed directly on the large primary producers. 
Consequently, there is a high biomass of top-level predators in upwelling 
systems or other highly productive areas. Figure 5.2 compares biomass and 
length of food chains in regions of high and low productivity. 

QtJESTION 5.5 Refer to Figure 5.2. (a) How many orders of magnitude 
difference are there i ti the amounts of biomass produced in the Antarctic 
Ocean and in the equatoriaJ Pacific? (b) Whal factors contribute to this 
difference? (Refer al so to Figure 5.3 and Seetion 3.5.) 

For a given locality, the number of trophic levels can be coupled with a 
quantitative estimate of primary productivity to predict yields of secondary 
production in any particular trophic level (P( n + 1 >), according to the following 

P {n + X ) = P\E n 


In this equation, P\ is annual primary production, E is the ecological 
efficiency, and n is the number of trophic transfers (which equals the 
number of trophic levels minus 1). A major difficulty in applying this 
equation lies in the accuracy of the valúes used for ecological efficiency and 
number of trophic levels. For example, the secondary production estimate 
can increase by an order of magnitude if the valué for E is doubled. Until 
we can be more confident of the valúes for ecological efficiency and the 


number of trophic levels in different locations, predictions of potential fish 
catches based on this method remain uncertain and unreliable. Nevertheless, 
equation 5.2 provides relative valúes that are useful for comparing 
production in different marine areas. 

QUESTION 5.6 ]f the primary producto vity of a Coastal urea is 
300 g C m -2 yr _l and herring (whieh feed on zooplankíon) are the principal 
fishery. what would be the expected annual máximum yield of herring (in 
terms of g C m _2 h given an average ecológica! effieieney of I0%? 

Table 5.1 couples general valúes for primary productivity (from Section 3.6) 
in the three major pelagic habitats (Figure 5.3) with numbers of trophic 
levels, and leads to the conclusión that upwelling regions should produce by 
far the highest numbers of fish (or whales) per unit area, and the open ocean 
the fewest. Even correcting for percent ocean area, the small upwelling areas 
should produce four times more fish than Coastal areas. In fact, upwelling 
areas provide a significant fraction of the present world fish catch. Remember 
too that there is a major economic advantage in catching fish in upwelling 
and coastal areas because large numbers can be harvested within 30-80 km 
of the coastline. In contrast, the expense of commercial harvesting is much 
higher in the open ocean because the stocks are dispersed over a vast región. 


In reality, the concept of a food chain is a theoretical convenience and an 
attempt to reduce a complex natural system to simple dimensions. There are 
seldom simple linear food chains in the sea. Practically all species of 
organisms may be eaten by more than one predatory species, and most 
animáis eat more than one species of food. The energy system is more 
accurately portrayed as a food web with múltiple and shifting interactions 
between the organisms involved. Many species do not conveniently fit into 
the conventional trophic levels. Some species are omnivorous, feeding on 
both phytoplankton and zooplankíon. Some also feed on detritus, the 
organic debris of faecal material, plant and animal fragments, crustacean 
molts, and abandoned larvacean houses and pteropod feeding webs. Some 
species change diets (and trophic levels) as they grow, or as the relative 
abundance of different food items changes. Still other species are parasites 
and obtain their energy from their hosts, and cannibalism is not uncommon 
within many marine species. Further, the benthic food chain is also linked to 
the pelagic production, as is illustrated in the continental shelf habitat in 
Figure 5.3. Some benthic species (e.g. barnacles, mussels) feed directly on 
phyto- or zooplankton, and other benthos are indirectly dependent on the 
pelagic production. 

There are typically fewer species in high latitude communities, and polar 
marine food webs tend to be simpler than those of other localities. For this 
reason, we have chosen to present a schematic depiction of the food web of 
the Antarctic Ocean in Figure 5.4. Note that in this area, as well as in the 
Arctic Ocean, there are two basic types of primary producers: the pelagic 
phytoplankton and the algae which live within the ice. The latter, called 
epontic algae, are generally benthic species that are adapted for the low light 
intensities present at the undersurface of the ice. The abundant krill form the 
central point of the Antarctic food web as they are the dominant herbivores, 


Figure 5.4 A diagrammatic representation of the food web in the Antarctic Ocean. 

and they are an important food source for several species of carnivorous 
zooplankton, pelagic fish and squid, as well as for plankton-feeding baleen 
whales, seáis, and seabirds. The abundance of krill may best be illustrated by 
pointing out that a single blue whale may consume up to eight tonnes (or 
more than 40 million individuáis) of euphausiids daily during summer 
months. When several species rely heavily on a single food, as illustrated in 
Figure 5.4, competition for food may develop between the different 
predators if their shared food becomes a limiting resource. The decimation 
of Antarctic baleen whales by commercial whaling has demonstrated how 
the flow of energy through a food web may be altered when a competing 
species is eliminated or reduced. Table 5.2 presents the relative amounts of 
krill consumption by different predators before and after depletion of whale 
stocks. A large decline in baleen whale biomass made more of the euphausiid 
biomass available to competing species, and consequently increased the 
populations of Antarctic seáis and birds by about a factor of three. 

Competitive relationships among species may also diminish the biomass of 
top-level commercially-fished species. Larval fish, for example, may 
compete with chaetognaths, jellyfish, ctenophores, and other invertebrate 
carnivorous zooplankton for copepods or other prey. Consequently, only a 


Table 5.2 Estimated changes in pattems of consumption of Antarctic krill by the 
major groups of predators, 1900-84._ 



Annual krill consumption 
(tonnes x 10 6 ) 


Baleen whales 


















certain fraction of the prey (and ultimately of the primary production) is 
converted to fish stocks. Competition for food and the resultant loss of 
energy to higher trophic levels is illustrated schematically in Figure 5.5. The 
same planktonic invertebrate predators also may eat the meroplanktonic 
larvae of the benthos, and thus they can decrease the production of shellfish, 
such as mussels and clams, as well as of noncommercial benthos. 

Although food webs are more realistic than food chains, they are more 
difficult to quantify in ecological terms. Few marine Systems have been well 
enough studied to attempt an energy budget analysis, in which the initial, 
measured, energy input of primary production is channelled into different 
trophic levels and pathways of the food web. The North Sea, however, has 
been intensively studied because of its long-time importance as a fishing 
región, its relatively small area, and its proximity to centres of marine 
research. Figure 5.6 outlines a quantitative analysis of energy flow through a 
food web of the North Sea based on major groups of organisms, rather than 
individual species. 

Food Chain 

Food Web 

Figure 5.5 A comparison of a hypothetical 
marine food Chain and food web. Both begin 
with 100 arbitran/ units of phytoplankton 
primary production. The food Chain produces 
0.2 units of fish from this primary production, 
but the food web produces only one-half this 
amount of fish. In the food web, two 
carnivorous species (A and B) compete for the 
supply of herbivorous zooplankton. Half of the 
herbivores are consumed by Species A and half 
by Species B. Fish do not eat Species B, so 
their principal food supply is less by 50%. 


Figure 5.6 A North Sea food web. Numbers 
reler to annual production in g C m~ 2 . 

The food web in Figure 5.6 is based on a primary productivity of 
90 g C m~ 2 yr' 1 , and it assumes (probably unrealistically) that the 
zooplankton consume all of the phytoplankton and excrete about 30% of 
their food as faeces. The pelagic herbivore production of 17 g C m~ 2 yr -1 
has been estimated from field and experimental studies on the copepod 
Calanus finmarchicus, which is the dominant planktonic herbivore in the 
System. The production valúes for the next trophic level come from 
assuming that 50% of the herbivore production is eaten by planktonic 
invertebrate camivores (such as chaetognaths and ctenophores) and 50% by 
fish. This equal allocation of energy is assumed, however, to result in lower 
production valúes for fish (even though they may additionally consume some 
invertebrate camivores) because they have higher energy demands and burn 
a higher percentage of their food in respiration than do invertebrates. The 
partitioning of energy through the sinking of faeces, their decomposition by 
bacteria, and the incorporation of bacterial production into benthic 
invertebrates has been based on similar assumptions. The predicted yield to 
humans in terms of fish catch is less than 0.7 g C m~ 2 yr' 1 , which is about 
0.8% of the plant production. 

Although the valúes of production in Figure 5.6 are extremely tentative and 
based on numerous assumptions, the model provides a scheme that links 
pelagic and benthic production and is an attempt to quantify a complex 
ecological system. Despite the difficulties, there are many potential rewards 
in studying food webs. Such studies allow us to examine the interactions of 
nutrient input, primary production, and secondary production, and to 
discover the determinants of production. They also can provide answers as to 
why there are particular patterns of species associations and of energy flow, 
and how these persist over time. It is also important to know how food webs 
will react to perturbations, such as pollution and commercial harvesting of 
high trophic levels. 



The regeneration of nutrients in the sea is a vital part of the interaction 
between higher and lower trophic levels. This is accomplished by bacteria 
and planktonic protozoans interacting in a microbial loop that is coupled 
with the classic food chain formed by phytoplankton-zooplankton-fish 
(Figure 5.7). Particulate detritus formed through natural mortality of phyto- 
and zooplankton and nekton, or through the production of faecal pellets and 
structures such as crustacean molts, abandoned pteropod feeding webs or 
larvacean houses, is decomposed by bacteria which utilize the energy-rich 
detritus for growth. Bacteria also can utilize soluble organic materials 
released by the physiological processes of animal excretion and 
phytoplankton exudation, thereby efficiently converting dissolved nutrients 
into particulate biomass. Thus the microbial loop is of particular importance 
in increasing food chain efficiency through utilization of both the very 
smallest size fractions of particulate organic material (POM), as well as of 
the dissolved organic matter (DOM) which is usually measured as dissolved 
organic carbón (DOC). 

The number of bacteria in the euphotic zone of the oceans is generally 
around 5 x 10 6 mi -1 . They may sometimes increase to 10 8 mi -1 in the 
presence of adequate nutritive materials and in the absence of bacterial 
grazers. In deep ocean waters, bacterial numbers may be less than 10 3 mi -1 . 
The number of bacteria in the sea is generally controlled through predation 
by nanoplankton, especially by various protozoans, but a few larger 
zooplankton (e.g. larvaceans) are also capable of capturing and consuming 

Figure 5.7 A schematic illustration showing the coupling of the peiagic grazing food chain 
(phytoplankton to piscivorous fish) and the microbial loop (bacteria and protozoans). Dashed 
arrows indícate the release of dissolved organic material (DOC) as metabolic by-products. 
The DOC is utilized as a source of carbón by heterotrophic bacteria. The bacteria are 
consumed by protozoans, which in turn are eaten by larger zooplankton. 


10 5 r. 

~l lililí- 1 I 1 nuil_I » 1 I _ I 1 i lililí I I ni 

0.03 0.1 1.0 10 100 

chlorophyll a{\x g I' 1 ) 

Figure 5.8 The relationship between 
chlorophyll a and bacterial abundance in the 
euphotic zone. Data are from •, central North 
Pacific gyre; o, Southern California Bight and 
Santa Monica Basin; and various additional 
marine (a) and freshwater (□) locations. 

bacteria (see Section 4.2). Among the nanoplankton, the zooflagellates are 
particularly voracious in their consumption of bacteria. Heterotrophic 
zooflagellates are usually present in concentrations of about 10 3 mi- 1 . 
However, when bacterial numbers start to increase, the zooflagellates quickly 
respond by consuming more bacteria and multiplying at a rate that tends to 
prevent very large increases in bacterial standing stock. Small bactivorous 
zooplankton are important links in transferring bacterial production to higher 
trophic levels as they, in turn, form a source of food for larger organisms, 
particularly for filter-feeding crustaceans. In general, filter-feeders like 
copepods and euphausiids are incapable of feeding directly on bacteria 
because their filtering appendages are too coarse to retain such small 

0UESTI0N 5.7 In addiüon to heterotrophic zooflagellates and larvaceans. what 
other zooplankton are eapable of feeding on bacteria? (Rder to Section 4.2. ) 

The general pathways of the recycling processes described above are shown 
in Figure 5.7, where it is apparent that bacterial activity in the sea is closely 
linked with marine food webs. Changes in the phytoplankton standing stock, 
in particular, are often closely accompanied in time and distance by changes 
in bacterial biomass. This is illustrated in Figure 5.8 which demonstrates that 
as the chlorophyll concentration increases from about 0.5 ¡xg to 100 ¡xg l -1 , 
bacterial densities increase from about 10 6 mi -1 to 3 x 10 7 mi -1 . During 
exponential growth of the phytoplankton, bacteria can live off dissolved 
organic metabolites (exudates) that are released as part of the metabolic 
processes of phytoplankton growth. At the end of a phytoplankton bloom, 
when the algae enter a senescent stage, there is an accumulation of 
phytodetritus (i.e. nonliving particulate matter derived from phytoplankton) 
and an increased release of dissolved metabolites. It is particularly at this 
time that the bacteria can utilize these energy sources to multiply and 
produce a sharp pulse (or bloom) that follows the phytoplankton bloom. 

Thus the food web of températe seas may shift seasonally from one that is 
based on high nutrients, diatoms, and filter-feeding copepods, to one that is 
dominated by the microbial loop and bactivorous zooplankton. A similar 
relationship between phytoplankton and bacteria influences the vertical 
distribution of bacterioplankton. Máximum numbers of bacteria generally 
occur at the pycnocline, where phytodetritus accumulates by sinking from 
the overlying euphotic zone. There, decomposition by bacteria contributes to 
the formation of oxygen minimum layers in stable waters (see Section 4.4). 
In general, it is estimated that bacteria, by using phytodetritus or dissolved 
organic exudates for their growth, may utilize up to 50% or more of the 
carbón fixed by photosynthesis. 

Note in Figure 5.8, however, that the relationship between increasing 
phytoplankton and increasing bacterial numbers holds least well at very low 
chlorophyll concentrations (<0.5 ¡xg l -1 ), where bacteria are more numerous 
than expected. This indicates that, in very oligotrophic waters, bacteria 
constitute the dominant biomass of the microflora, and their numbers are 
independent of the very small amount of phytoplankton. In waters where 
nutrient concentrations are very low and limiting, there may be competition 
between bacteria and phytoplankton for essential elements. In this 
circumstance, predation on the bacteria by protozoans may influence the 
outcome of the competition. 


QUESTIQN 5.8 In Figure 5.8, why are ihe valúes for chlorophylt a and 
bacieriaJ abundance lowesi in the central North Pacific gyre and highest in 
fres h w ater I oca t i o n s ? 

Marine viruses are the smallest and most abundant organisms in the sea, 
with concentrations ranging between about 10 3 and 10 9 mi -1 , yet their role 
in the microbial loop and generally in marine ecosystems remains highly 
speculative. There are significantly more viruses in near-surface waters 
compared with deeper layers, suggesting a coupling of viral particles with 
other upper-ocean biological processes. It is known that viral pathogens can 
infect marine bacteria and a variety of phytoplankton including diatoms and 
Cyanobacteria, and experimental work suggests that viral infection may 
affect the species composition of the phytoplankton community and 
significantly reduce primary productivity. 

Studies of the microbial loop are relatively new in the Science of biological 
oceanography. They have been hampered by the very small size of the 
microbes and protozoans and associated difficulties in collection, 
preservation, and identification. There is a need to understand the impact of 
this cycle on primary production in terms of nutrient competition and 
nutrient recycling, and on secondary production in terms of providing a link 
between bacterial production and its consumption by higher trophic levels. 



It is possible to obtain reasonable estimates of the amount of primary 
production in different marine areas (Section 3.2.1), and catch statistics of 
commercially fished species provide a minimum valué of production at the 
other end of food chains. Information about secondary production in the 
intermediary trophic levels and for noncommercial top-level predators (i.e. 
jellyfish, ctenophores, Trash fislT) is much more difficult to obtain. There 
are, however, compelling reasons to tackle this problem. In some 
circumstances, primary production may not be a good indicator of 
production in higher trophic levels. For example, in highly eutrophic systems 
(see Section 3.4), the growth of phytoplankton may greatly exceed what can 
be consumed by herbivores, or the phytoplankton species which becomes 
dominant under these conditions may not be a suitable food source for 
herbivores; in either case, much of the primary production may enter the 
microbial-detritus circuit instead of the classic pelagic food chain. Relying 
entirely on fish catch statistics to provide valúes of secondary production in 
top trophic levels leads to underestimates, because it omits production of all 
the competing, unharvested species. 

Secondary production can be estimated from field data. The production of a 
population of zooplankton is defined as the total amount of new biomass of 
zooplankton produced in a unit of time, regardless of whether or not it 
survives to the end of the time period. In this definition, biomass (B) is: 

B = Xxw (5.3) 

where X is the number of individuáis in the population and vv is the mean 
weight of an individual. It then follows that production (P t ), during a time 


interval from t\ to t 2 , can be expressed as: 

VVi + w 2 

P t = (X i -X 2 ) — 2 + (5.4) 

where the subscripts 1 and 2 indícate valúes obtained at time t\ or time t 2 , 
respectively. The expression (#2 — # 1 ) represents the increase in biomass 
observed during the time interval; the remainder of the equation (i.e. the 
decrease in population number times the average weight of an individual) 
represents an estímate of the biomass produced, but then lost through 
predation or water movements. 

Ideally, one would wish to follow changes over time in numbers and growth 
under natural conditions in a single cohort of a population, a cohort being 
one identifiable generation of progeny of a species. However, these 
conditions can seldom be met in the sea and, in any event, it is usually 
impossible to follow and sample the same water mass for a period of time 
long enough to obtain meaningful measures of growth. The best that can 
usually be achieved is to attempt to follow changes in relative numbers and 
weights of distinctive life stages of abundant species. Because many copepod 
species are dominant members of the plankton and have easily identifiable 
age classes, this group of crustaceans is often selected for production 
measurements, and the example given below is for a copepod species 
producing one generation per year. 

Figure 5.9 presents a hypothetical representation of the numbers of 
individuáis in different, successive developmental stages of a copepod, 
ranging from newly hatched nauplii through copepodite stages I, III, and V 
(see Section 4.2). Note that the numbers (X) change with time due to a 
number of natural processes including mortality, aggregation, or water 
exchange. In order to calcúlate production, we also need to calcúlate changes 
in weight, or growth. This can be done if the time interval between the 

Figure 5.9 A hypothetical representation of changes in the numbers of individuáis in 
selected successive developmental stages of a copepod having one generation per year. 

Some stages are difficult to distinguish from each other, so for that reason all naupliar 
stages (NI-NVI) are lumped together and only copepodite stages Cl, Clll and CV are 
considered here. (Refer to Section 4.2 for a discussion of copepod life cycles.) Wt and w 2 
indícate average weights of copepodite stages Cl and Clll, respectively. 



occurrence of máximum numbers of successive developmental stages is 
known, and the average weight of each stage is determined in the laboratory. 
In this example, there is a duration of 44 days between copepodite stage I 
and copepodite stage III, and a change in average weight from 0.15 mg to 
0.60 mg between these respective stages. Assuming that there were, on 
average, 80 stage I copepodites m -2 and 30 stage III copepodites m -2 , we 
now have the information needed to calcúlate production. 

QUESTK0N 5.9 Given i he Information above, and mmg equation 5.4. whai 
was the average produeiion per day of the copepod population during the 
44-day period between peak numbers of stage 1 and lll copepodites? 

It is important to note that the valué of P t for any population will change 
with time because zooplankton grow at different rates during their Ufe 
eyeles. Production will generally be positive during periods of máximum 
growth; in températe regions, P t would be highest in spring when food is 
abundant and most of the zooplankton are young. The valué for P t may be 
negative in winter months when food concentrations are low and animáis 
cease growing, or even lose weight. Because of these natural changes in P t , 
growth increments need to be determined for a number of points in the life 
history of a species in order to calcúlate annual secondary production. Thus, 
annual secondary production (P t ) becomes a summation of production 
calculated for successive time intervals (P t i, Pa , etc.), so that: 

Pt — Pt\ + Pt2 + Pt3 • • • Pti' (5.5) 

The method described above may only be applied to species with distinctive 
life stages, and it is only practicable in regions where young are produced 
seasonally. Particularly in warm waters, many marine animáis (including 
copepods and other Crustácea) reproduce more or less continuously, and thus 
there is a continuous input of young and a mixture of age and size classes. 
Further, many planktonic species do not have distinctly different life stages, 
and size or weight may not necessarily be good indicators of age. A good 
example of this is found in the thecosomatous pteropods (refer to 
Section 4.2); very young individuáis quickly produce an adult-sized shell, 
but the body grows only gradually to occupy the entire inner space. Whereas 
crustaceans have a determínate growth pattern in which growth in each 
stage is limited by size of the exoskeleton, many of the other zooplankton 
have indeterminate growth and are capable of more or less continual 
growth during favourable conditions, or they shrink (lose biomass) during 
periods with low food concentrations. Often the attendant difficulties of 
working with population data collected from the field lead researchers to 
work with populations or individuáis under experimental confinement, where 
conditions can be controlled and where individuáis can be studied over 
prolonged periods of time. Various experimental approaches to secondary 
production studies are discussed in the following section. 


Because the oceans are so vast and the waters are in continual motion, and 
because of the reasons outlined at the end of the previous section, it is 
necessary to study some biological oceanographic processes under 
experimental conditions. 


Many options are open to the experimentalist and some examples of 
different approaches are discussed below. In general, however, a choice can 
be made between: 

(1) laboratory-scale experiments, which tend to study individual 
organisms in relatively small volumes of water; 

(2) enclosed ecosystem experiments, which are carried out in very large 
containers of natural seawater in order to test the interactions of several 
trophic levels and their responses to perturbations; and 

(3) Computer model simulations, in which complex biological processes 
and ecological interactions can be studied on a Computer, including the 
influences of physical and Chemical environmental parameters on 
biological processes occurring in the sea. 

No single method of study is likely to provide all the answers required, and 
all experimental and observational approaches, from laboratory test-tube 
studies to satellite data-gathering, have both advantages and limitations to 
the type of results that are generated. Figure 5.10 illustrates schematically 
how the three experimental approaches outlined above can interact with field 
studies to lead to a better understanding of natural events. Data gathered 
from the field and from experimental studies can be fed into mathematical 
Computer models which attempt to intégrate this information and simúlate 
real conditions and events. These experimental options apply to studying 
problems in both pelagic and benthic marine ecology, but the examples 
discussed below concéntrate on issues in planktonic ecology. 

Laboratory experiments 

Experiments in the laboratory can be conducted to determine food 
requirements of zooplankton and transfer efficiencies between trophic levels. 
The majority of such experiments have been carried out using crustaceans, 
principally copepods, because they can be easily captured in large numbers 
and with little or no damage, and they are also amenable to laboratory 
culture. The majority of experiments have also been done with herbivorous 
species, partly because their phytoplankton food can be cultured easily. A 

Figure 5.10 The interaction of field studies, 
experimental studies, and Computer modelling 
in biological oceanographic research. 


few carnivorous zooplankton, including certain chaetognaths and 
gymnosomes, have been utilized for experimental research but, in these 
cases, their zooplankton prey must also be amenable to culture or, 
alternatively, the prey must repeatedly be collected fresh and with minimal 
damage from the sea. The same principies and equations, using modified 
techniques, can be employed to study production in benthic animáis. 

Experimental studies of feeding and secondary production are based on the 
premise that only a certain fraction of the energy ingested in food can be 
utilized for production. The remaining fractions are expended in respiration 
or excretion, or are never utilized and pass through the animal in faecal 
material by the process known as egestion. The fractionation and utilization 
of energy can be expressed as: 

G = R-E -U-T (5.6) 

where growth (G) is a measure of secondary production; R is the ration of 
ingested food; E is egested faecal material; U refers to excretory products 
(e.g. ammonia, urea); and T represents respiration. The units in this energy 
balance equation are given in joules or calones per unit weight. Excretory 
products are usually considered as a negligible fraction of the equation, and 
the equation can be simplified and rewritten as: 

AR — T G (5.7) 

where R , T and G are as defined above, and A is a constant relating the 
proportion of food assimilated (or actually utilized) to the amount consumed. 
AR is thus referred to as the assimilated ration. The rationale for establishing 
equations 5.6 and 5.7 is presented schematically in Figure 5.11. 

Figure 5.11 A schematic división of the losses 
and uses of the energy contained in consumed 

Feeding experiments usually involve introducing from one to several tens of 
animáis (depending on their size) into incubation beakers filled with a 
measured volume of seawater containing a known concentration of food 
partióles. Controls are established in bottles containing food but no animáis; 
these will show any changes in food concentration that occur independently 
of grazing or predation and will allow an assessment of error in counting 
food organisms. In the case of grazing experiments with filter-feeding 
herbivores, culture bottles are kept dark to minimize growth of the 
phytoplankton. Usually it is necessary to introduce a mechanical means of 
keeping cells in suspensión and randomly distributed in the water. After 
containers have been incubated for a measured time interval, the food 
concentration is again determined. This can be done visually with a 
microscope, or electronic particle counters may be used if the particles are 
small and of appropriate shape. 

There are a number of ways to calcúlate the grazing rate (number of algal 
cells eaten per herbivore per hour or day) or the predation rate (number of 
prey animáis eaten per carnivore per hour or day). Both of these rates 
become more meaningful for comparative purposes if they are converted to 
ingestión rate, which is the weight or energy content of food ingested per 
animal per hour or day; in this case, weight can be expressed in terms of dry 
weight, organic matter, carbón, or nitrogen. Many factors affect feeding rates 
including temperature, type of food, and concentration of food. There is 
usually a direct correlation between the amount of food eaten and the food 
concentration, the relationship being expressed as: 

R = KmaxO - e~ kp ) 



where R is the radon of food ingested at a food or prey density /?, R m . dX is 
the máximum radon taken at sadation, and A: is a grazing constant linking 
food concentradon and ingestión. Thus the radon increases with food 
concentration to some máximum level, as shown in Figure 5.12. 

In order to establish the proportion of food that is actually digested in the gut 
and assimilated by an animal (A/? in equation 5.7), an assimilation efficiency 
is calculated by comparing the ingested food with a quantitative measure of 
faeces produced. Thus, assimilation efficiency (A) can be calculated from: 

A = 



x 100% 


where R is the amount of food (radon) ingested and E the amount of faeces 
produced. (Note that in equation 5.7, A is the assimilation efficiency given in 
decimal form.) Although assimilation efficiency varies with type of food, age 
of an animal, and other factors, it is generally high ( ca . 80— > 90%) for 
carnivorous animáis, and somewhat lower (ca. 50-80%) for herbivores. 
Detritus feeders have the lowest assimilation efficiencies, usually being less 
than 40%. 

Why are assimilation efficiencies difierent in carnivores* herbivores, and 
detri livores? 

Assimilation is high in camivores because of the similar biochemical 
composition of the prey and predator. It is lower in herbivores because of 
the greater difficulty in digesting plant food, particularly carbohydrates and 
especially cellulose. Assimilation efficiency is lowest in animáis that feed on 
detritus because much of the food they consume, such as skeletal 
components, is indigestible. 

Figure 5.12 The relationship between the 
amount of food ingested (R) and the 
concentration of food available (/?). Note that R 
increases up to a certain food concentration, 
then remains at the same level (/? max ) with 
further increases in food. 

Valúes for respiration losses (T in equations 5.6 and 5.7) can be determined 
in the laboratory. Bottles with and without experimental animáis are 
prepared simultaneously with known volumes of seawater. Changes in 
dissolved oxygen concentration after an incubation period are detected by 
Chemical methods or oxygen electrodes, and are attributed to respiration of 
the animáis. Respiration rates are directly correlated with the environmental 
temperature, but they are inversely correlated with size of an animal, being 
higher per unit weight in smaller animáis. In general, 40-85% of the 
assimilated food (AR) will be utilized for metabolic maintenance. 

Hovv would difieren! respective respira! ion mies affect the daily food raí ion 
i>f sma11 zooplankton compared with 1 urge zoopIankton? 

Because small zooplankton have higher respiration rates per unit body 
weight, they require much higher food intake relative to body weight. Very 
small zooplankton (e.g. crustacean nauplii) may require and ingest food 
equivalent to more than 100% of their body weight per day. In contrast, 
large zooplankton (e.g. adult euphausiids) may eat the equivalent of only 
about 20% of their body weight per day. 

Referring to equation 5.7, growth or production (G) of zooplankton can be 
calculated indirectly from valúes obtained for ingestión rates (/?), 
assimilation efficiency (A), and metabolic losses (T). In some circumstances, 
growth can be measured directly in laboratory experiments by measuring or 
weighing experimental animáis at successive time intervals. There are two 
expressions that link the growth (G = AW/At) of an animal with the 


amount of food ingested ( R ) by that animal. One is known as the gross 
growth efficiency, K\, which is expressed as: 

K\ = G/Rx 100%. (5.10) 

The other expression is the net growth efficiency, K 2 , which is a ratio of 
growth (G) to assimilated ration (Afí), so that: 

K 2 = G/ARx 100%. (5.11) 

QUESTIQN 5,1 B The gymnosome Clione limad na, feeding orí the shelled 
pteropod Umaana hethiruL inereased in dry weight by un average of 5.0 mg 
per month, while consuming an average of 7.5 mg dry weight of prey. 

Clione assimilates its prey with 90% efficiency, (a) What is the gross growth 
efficiency for Clione? (b) What is the valué of K 2 for ibis gymnosome? 

Temperature and food concentration will influence both K\ and K 2 valúes, 
and both valúes change with age of an animal. The number of eggs produced 
by experimental females may also be included as parí of the total production 
in energy balance equations and, in general, fecundity increases with 
inereased food ration. The efficiency at which assimilated food is converted 
into growth or progeny (K 2 ) usually ranges between 30% and 80% for 
zooplankton and fish. This is much higher than growth efficiency in 
terrestrial mammals, which tends to be on the order of 2-5%. Some of the 
difference arises because poikilothermic animáis (planktonic invertebrates, 
fish) have lower metabolic costs than warm-blooded mammals. As well, 
terrestrial mammals expend energy in fighting the effeets of gravity, whereas 
pelagic species are buoyed up in their seawater environment. 

Experimental studies of energy requirements and energy partitioning as 
described above provide a more realistic basis for formulating ecological 
principies that govern food chains in the sea. These types of experiments 
permit us to judge the levels of food concentrations required to produce a 
certain number of animáis at different trophic levels, at least under the 
conditions maintained in the laboratory. Conversely, knowing the amount of 
primary productivity in any particular marine región, and applying energy 
budget valúes obtained from the laboratory, permits a better prediction of the 
amount of secondary production that might be expected under those 

The laboratory approach to establishing energy budgets in zooplankton has 
given us useful comparative valúes for different types of animáis, and has 
allowed an assessment of transfer efficiencies based on ecological trophic 
theory and field surveys. Nevertheless, it is important to keep in mind that 
only some species are amenable to laboratory culture; the conditions are 
always artificial: and the results must be extrapolated to natural situations, 
including natural foods and changing environmental conditions. 

Enclosed experimental ecosystems 

An enclosed experimental ecosystem is a system in which a large volume of 
natural seawater is artificially enclosed in order to study populations of 
phytoplankton and zooplankton over time. This alleviates the problem of 
studying plankton populations at sea, where they are being continually 
displaced from one location to another by water movement. If the 
experimental system is large enough, it may be possible to study the 


interactions of several trophic levels, including such planktivorous species as 
ctenophores and fish. 

Several conditions must be met in setting up an enclosure. In order to contain 
sufficient plankton and a reasonable semblance of the natural environment, 
the volume of seawater captured must usually be between 100 and 1000 m 3 . 
It must be enclosed in such a way that natural sunlight can penétrate the 
water column. The depth profile of the water column in terms of temperatura, 
nutrients, and salinity must also be preserved, and care must be taken that no 
toxic substances are accidentally introduced. These conditions can usually be 
achieved by using moored, transparent containers made of nontoxic material. 

Figure 5.13 illustrates enclosures that were moored in a fjord in British 
Columbia, Cañada, and used in studies known as the Controlled Ecosystem 
Pollution Experiments (CEPEX). Each 30-m deep container enclosed about 
1300 m 3 of seawater. The purpose of the programme was to test the effects 
of small traces of pollutants on the ecology of the plankton community and 
on young fish. The employment of several enclosures permitted the addition 
of different types of pollutants in differing concentrations, as well as 
providing a control enclosure that could be compared with conditions in the 
surrounding, unenclosed water. Experiments were conducted with traces of 
copper (10-50 ppb), mercury (1-10 ppb), petroleum hydrocarbons 
(10-100 ppb), and several other potential pollutants that can be found in 
seawater, particularly in Coastal areas. 

Figure 5.13 An artist’s illustration of 
controlled experimental ecosystem enclosures 
floating in the sea. Each polyethylene container 
is 30 m deep and holds about 1300 m 3 of 
seawater. The scuba divers are drawn to 
approximate scale. 

Figure 5.14 provides an example of the results that were obtained in one 
experiment. At the start, diatoms ( Chaetoceros ) were the dominant primary 
producers in all enclosures, and they remained dominant in the control 
containers (J and K) throughout the experimental period. In the experimental 
ecosystems treated with copper (L and M), most of the diatoms died within 
three weeks (Figure 5.14a) and were replaced by small photosynthetic 
flagellates that began to increase after the first week following pollutant 
addition (Figure 5.14b). This change in the structure of the primary 
producers from large diatom chains ( ca . 500 (x m) to small flagellates of less 
than 20 /im is analogous, but on a microscale, to a change in size of 
terrestrial vegetation from trees to grass, and it had a profound influence on 
the total ecology of the enclosures. Changes in the phytoplankton 
community were accompanied by changes in the dominant species of the 
zooplankton community and, after about one month, the copper-treated and 
control containers had very different ecologies. This kind of result is almost 
impossible to obtain from either laboratory-scale experiments (which are too 
small) or from field experiments (where the water moves around too much). 

QUESTI0N 5,11 Figure 5.14 shows that a small amount of copper ¡n seawater 
caused a change in the primary producers, with long-chain diatoms being 
replaced by small flagellates, What difference might this make to the rest of 
the food chain in terms oí the type of dominant species and nurrtber of 
trophic levels? (Refer to Section 5,1 for help ií necessary.) 

Enclosed ecosystem experiments have been performed at many different 
locations including in Loch Ewe in Scotland; at the Marine Ecosystem 
Research Laboratory in Rhode Island, U.S.A.; and in Xiamen in the People’s 
Republic of China. Not all of the experiments have involved pollutants. 

Some have been designed to test the effects of natural perturbations, such as 
changes in light levels and in amount of vertical mixing, on plankton 


Figure 5.14 The results of experiments in enclosed experimental ecosystems. (a) The 
percentage of long-chain diatoms (Chaetoceros) in the total phytoplankton. (b) The 
percentage of small flagellates in the total phytoplankton. L and M, copper-treated 
containers; J and K, control containers. 

ecology. Such studies provide valuable insights into the influence of physical 
variables on biological processes. 

The disadvantages of using enclosed experimental Systems of this type 
should also be mentioned. Perhaps the greatest drawback is the removal of 
small-scale physical turbulence from enclosed waters. Mixing is an 
important property of natural marine communities, and the damping out of 
turbulence within the containers can lead to spurious results if the 
experiments are continued for too long a time. Also, some caution has to be 
exercised in applying results obtained in experiments in one area to other 
locations. The waters of Loch Ewe, Scotland, for example, are not the same 
(physically, chemically, or ecologically) as those in the South China Sea, 
and similar experimental procedures may produce quite different results in 
the two localities. 

Computer simulations of marine ecosystems 

Computer models provide a third way of generating knowledge about the 
way marine ecosystems opérate. Data gathered from many sources (e.g. field 
measurements, laboratory or controlled ecosystem enclosure experiments, 
satellite imagery) are entered into mathematical models designed either to 
simúlate natural events or to provide predictions of future events. The type 
of model used depends on the questions being asked. A relatively simple 
model can be used to predict an oxygen balance in a particular body of 
water, but more complex models are required to examine trophic interactions 
between several levels in marine food webs. 

The usual approach in setting up Computer models that will examine 
ecological issues is to formúlate a number of differential equations made up 
from non-linear empirical relationships which connect various forcing 
functions with ecotrophic structure (Figure 5.15). For example, the amount 
of available light and the concentrations of nutrients would be considered 
forcing functions in such equations; they set the constraints on production 
in the system being considered. A model would also consider non-linear 


Figure 5.15 Ecosystem Computer model 
construction showing examples of forcing, 
physiological, and phasing functions. 

1. Forcing functions 2. Physiological functions 

2.1. Primary productivity vs. light 

2.2. Zooplankton grazing on phytoplankton 

3. Phasing functions 

3.1. Extinction coefficient on light 

3.2. Temperature effect on growth 

physiological functions, such as the reaction of light with phytoplankton 
(see Section 3.3). In addition to these two types of functions, there are a 
number of other environmental influences which modify both the forcing and 
physiological functions. For example, changes in temperature will modify 
most physiological functions, and the light extinction coefficient will modify 
the forcing function of light. These modifying influences form a third type of 
function; they are called phasing functions because they tend to speed up or 
slow down the interactions between forcing and physiological functions. 

The layout of a simple Computer model involving the interaction of different 
trophic levels is shown in Figure 5.16. In this system, the growth rate of the 
phytoplankton is govemed by light and nutrients, The zooplankton graze the 
phytoplankton according to an equation in which the grazing rate is 
dependent on the concentration of phytoplankton; the one planktivorous 
species (a ctenophore) utilizes the zooplankton depending on the availability 
of the prey. The model also ineludes a reeyeling loop in which bacteria 
utilize dissolved organic material (DOC) lost from the phytoplankton and 
retum part of it to the system via zooflagellates and microzooplankton. This 
model can be run on a desk-top Computer, and an illustration of the kind of 
results that one can obtain are shown in Figure 5.17. 

GUESTIÜN 5.12 In the Computer model layout shown in Figure 5*16 which are 
(a) the forcing functions and (b) the physiological functions? 

Figure 5.17 shows the results of a Computer simulation designed to test the 
effect of changing only one parameter, the extinction coefficient of light, on 
the output of the model in terms of the amount of phytoplankton and 
zooplankton produced. The amount of light was decreased by increasing the 
extinction coefficient (see Section 2.1.2) from 0.2 to 0.3 and to 0.7 m" 1 . As 
one might expect, the amount of phytoplankton decreases during an 8-day 
period as the extinction coefficient increases. However, the standing stock of 
zooplankton increases with a decrease in light intensity from extinction 
valúes of 0.2 to 0.3 m -1 . This increase can be attributed to the fact that the 
phytoplankton grow more slowly at an extinction valué of 0.3 m -1 than at 
0.2 m _1 , and this in turn enables the slower-growing zooplankton to graze 
more of the phytoplankton, which would otherwise sink out of the water 
column as ungrazed phytodetritus. The very abrupt decline in the 
phytoplankton at day 4 (when k = 0.2) is due to nutrients becoming 
exhausted by the rapidly growing phytoplankton, and to the sinking of 
accumulating phytodetritus. The maximal zooplankton growth at a light 
intensity in between very bright and shady is a result of interactions within 


Figure 5.16 A simple trophodynamic model 
for use in Computer simulations of trophic 
interactions. DOC, dissolved organic carbón. 

the model, and could not be readily seen without the modelling approach. 
The use of ecosystem models has indicated that many other trophic 
relationships optimize at intermedíate environmental valúes. 

QUESIIOH 5.13 In the computar model resuíis shown in Figure 5,17, why is 
the pro Judión of zooplankton lowest vvhen k — 0.7? 

Computer simulation models have both advantages, as described in the 
example above, and disadvantages. They cannot replace the gathering of real 
data. Efforts can be made with models to simúlate actual events. More often 
than not, however, data generated by actual events are the result of so many 
variables that it is statistically difficult to tune any one model to explain 
them. Thus a model may more often be used to project different scenarios of 
likely events. When compatibility with actual field data is required, a 
biological trophodynamic Computer model must be coupled to a Computer 
model of the physical environment. Such models have been produced, and 
an example is the General Ecosystem Model for the Bristol Channel and 



Figure 5.17 A Computer model simulation 
predicting the production of phytoplankton 
(solid lines) and zooplankton (dashed lines) 
from different light extinction coefficients (Ür), 
(Standing stocks in relative units.) 


3 4 5 





Severn Estuary (known as GEMBASE) developed by the Institute for 
Marine Environmental Research in Plymouth, England. In this Computer 
model, an ecological submodel is operated in seven different physical 
domains, and water exchanges between these subregions are determined by 
two physical models describing water flow. Integrated water exchanges 
produced from the physical models are then entered as input into the 
slower-scaled ecological models. 


The preceding discussions of pelagic production and food webs have 
highlighted unique ecological features of the marine habitat. You may 
already have noted profound differences between marine and terrestrial 
ecology. The follgwing points summarize differences in primary and 
secondary production in these two major environments. You may wish to 
compile your own list of other differences in physical features of the 
contrasting environments and of general anatomical and behavioural 
differences in marine and terrestrial organisms. 

1 In the open ocean, the majority of the primary producers are microscopic 
phytoplankton (Sargassum weed being one of the few exceptions). Although 
macroscopic algae and sea grasses contribute to marine production in inshore 
shallow areas, by far the greatest amount of marine primary production is 
carried out by the small phytoplankton. In contrast, the great majority of 
terrestrial primary producers are large, highly visible forms like grass and 

2 The basic structure of phytoplankton and terrestrial vegetation is very 
different. The small size of phytoplankton enhances flotation and also, 
because of the surface to volume ratio, promotes uptake of nutrients directly 
through the cell wall from the surrounding water. Terrestrial plants, in 
contrast, require roots for anchoring and for nutrient uptake from the soil. 
They also tend to develop trunks and branches to fight gravity and maximize 
exposure to sunlight, and this requires the production of carbohydrates like 
cellulose and lignin which confer strength and rigidity. Phytoplankton, in 


contrast, do not require large amounts of structural carbohydrate and are 
largely composed of protein. 

3 Because of the small size and high protein content of the phytoplankton 
and their relatively low abundance compared to herbivore numbers, a major 
fraction of the marine primary production is usually consumed, digested, and 
readily assimilated by marine herbivores. Comparadvely little of the pelagic 
primary production goes directly into the decomposer cycle. This is not the 
case in the terrestrial ecosystem, where much of the primary production is in 
the form of inedible or indigestible structural components, such as cellulose 
and lignin, that are contained in bark, tree trunks, and roots. Terrestrial 
animáis rarely eat more than 5-15% of the total plant production, and they 
consume a lot of material that is largely indigestible. Much of the terrestrial 
photosynthetic production therefore enters indirectly into the food chain via 
the decomposition cycle. 

4 In the pelagic environment, primary productivity ranges from about 50 to 
600 g C m~ 2 yr -1 . In comparison, terrestrial primary productivity varíes 
from virtually zero in arid deserts and regions that are too coid for plants 
(Antárctica) to máximum valúes of about 2400 g C m -2 yr -1 in grasslands, 
and about 3500 g C m -2 yr -1 in tropical rainforests. Although primary 
productivity of benthic marine plants in shallow areas may approach 
terrestrial valúes, in general the primary production per unit area in the sea is 
much lower than on our ‘green’ land. 

5 A useful comparison can be made by examining the production to 
biomass ( P/B ) ratio; this is the relationship between the total annual 
production and the average biomass of living plant (or animal) material 
present throughout the year. Although the total biomass of the small marine 
phytoplankton may be relatively low, these algae are fast-growing. 
Consequently the P/B ratio for phytoplankton is roughly 100-300 in the 
marine pelagic ecosystem; this means that the phytoplankton biomass may 
turn over 100-300 times during a year. P/B ratios measured over one year 
are about an order of magnitude lower for marine zooplankton, and another 
order of magnitude lower for fish. In contrast, terrestrial plants have a very 
high total biomass (we can see this), but are generally slow-growing and 
long-lived; further, a large fraction of the primary production of these plants 
is used to maintain the respiration of that biomass. Therefore, terrestrial 
vegetation has much lower P/B ratios of about 0.5-2.0. 

6 Most of the animáis in the sea are cold-blooded (poikilothermic) 
invertebrates and fish and therefore have much lower energy requirements 
than terrestrial warm-blooded birds and mammals. As well, pelagic animáis 
live in a buoyant fluid and use little energy in locomotion. In comparison, 
mammals and birds (and even the larger poikilothermic insects) expend large 
amounts of energy fighting gravity; walking, crawling, and flying are all 
more energetically costly than swimming. This means that a relatively larger 
fraction of ingested energy can be channelled toward growth and 
reproduction in marine pelagic animáis. In fact, growth efficiency (the ratio 
of biomass production to quantity of food eaten) tends to be an order of 
magnitude higher in marine poikilothermic species. In contrast to land 
mammals and birds, they also produce larger numbers of young and usually 
expend no energy on parental care of the progeny. All of these features 
contribute to a much higher secondary production in the sea compared with 
that on land. 


7 Whereas plants so evidently domínate the biomass on land, animáis form 
the visually dominant group in the sea. Although the ocean contributes only 
about 50% of the world’s plant production, it accounts for more than 50% of 
its animal production. 


All elements that become incorporated in organic materials are eventually 
recycled, but on different time scales. The process of transforming organic 
materials back to inorganic forms of elements is generally referred to as 
mineralization. It takes place throughout the water column as well as on the 
bottom of the sea, where much of the detrital material from overlying waters 
eventually accumulates. Recycling of minerals may take place relatively 
rapidly (within a season) in the euphotic zone, or much more slowly (over 
geological time) in the case of refractory materials which sink and 
accumulate on the seabed. 

Figure 5.18 illustrates some of the ways in which elements are cycled by 
different groups of organisms. In the water column, where there is usually 
plenty of oxygen, decomposition of organic material takes place via 
oxidative degradation through the action of heterotrophic bacteria. Carbón 
dioxide and nutrients are returned for re-utilization by the phytoplankton. 

The cycle differs in anoxic areas where there is no free dissolved oxygen. 
Anoxic conditions are present in subsurface sediments on the seafloor and in 
a few special areas like the Black Sea, which is anoxic from about 200 m 
depth to the bottom because water exchange and mixing with the adjacent 
Mediterranean Sea are severely restricted by bottom topography. Under 
anoxic conditions, bacterial degradation takes place by anaerobic bacteria 
that utilize oxygen found in sulphate and nitrate radicáis. This type of 
oxidation forms highly reduced compounds such as methane, hydrogen 
sulphide, and ammonia. Since these compounds are high in Chemical energy, 
another group of bacteria (the chemoautotrophs) can utilize this energy to 
reduce carbón dioxide and make new organic material. The process of fixing 
carbón from CO 2 into organic compounds by using energy derived from 
oxidation of inorganic compounds (e.g. nitrite, ammonia, methane, sulphur 
compounds) is called chemosynthesis. 

Ecologically, the most important aspect of recycling in the sea is the rate at 
which growth-limiting nutrients are recycled. Among the nutrients that can 
be in short supply in the sea, nitrate (NO 3 - ), iron (bioavailable Fe), 
phosphate (PC> 4 3 ~), and dissolved Silicon (Si(OH) 4 ) are most often found in 
concentrations well below the half-saturation levels required for máximum 
phytoplankton growth (refer to Section 3.4). Silica limitation affects 
primarily those organisms that use this element to form skeletons; these 
inelude the diatoms and silicoflagellates among the phytoplankton (see 
Section 3.1), and the radiolarians among the zooplankton (refer to 
Section 4.2). The Silicon cycle is relatively simple as it involves only 
inorganic forms; organisms utilize dissolved Silicon to produce their 
skeletons, and this skeletal material dissolves following death of the 
organisms. The eyeling of phosphorus is also relatively simple in a Chemical 
perspective; at the usual alkaline pH of seawater, organic phosphate is 
relatively easily hydrolysed back to inorganic phosphate which is then 
available again for uptake by phytoplankton. Because phosphorus eyeles 
rapidly through the food chain, it is seldom limiting in the marine 


Figure 5.18 Marine organisms derive energy from light (photoautotrophs), from inorganic 
compounds high in Chemical energy (chemoautotrophs), from organic carbón compounds 
(heterotrophs) or, in the case of a few bacteria, from a combination of these processes. 
Each of these sources of energy may become limited at some time or place in marine 
habitáis. The supply of organic carbón compounds is largely limited by their rate of 
production from chemoautotrophs and photoautotrophs. Photoautotrophs are limited by the 
amount of light. Chemoautotrophs, which may be either aerobic or anaerobio species, are 
limited by the amount of highly reduced inorganic compounds derived from the metabolism 
of anaerobio heterotrophs. Thus each process is essentially dependent on the next process 
to recycle material through the entire system. 

environment. Compared with Silicon and phosphorus, the recycling of 
nitrogen, however, is a more complex process. 


The marine nitrogen cycle (Figure 5.19) is complex because nitrogen in the 
sea occurs in many forms that are not easily converted from one to another. 
These inelude dissolved molecular nitrogen (N 2 ) and the ionic forms of 
ammonia (NH 4 + ), nitrite (NO 2 - ) and nitrate (NC> 3 ~), as well as organic 
compounds such as urea (CCKNF^h)- The dominant form of nitrogen in the 
ocean is the nitrate ion, and it is often this form that is taken up by 
phytoplankton, although many species can also utilize nitrite or ammonia. A 
few phytoplankton species can also take up some small molecules of organic 
nitrogen, such as amino acids and urea. The rate at which nitrogen in a 
suitable State is made available for phytoplankton may limit primary 
production in oligotrophic waters throughout the year and in températe 


waters during the summer. Remember too (from Section 3.4) that iron is 
necessary for the formation of reducíase enzymes that are used in the 
conversión of nitrite and nitrate into ammonium, and this is used to make 
amino acids. If iron is present in limiting concentrations, then even an 
abundance of nitrate will not promote máximum phytoplankton production. 

Regeneration of nitrogen in the water column results from bacterial activities 
and excretion by marine animáis, especially the excretion of ammonia by 
zooplankton. As illustrated in Figure 5.19, the oxidation of ammonia to 
nitrite and then to nitrate is referred to as nitrification; the bacteria that 
mediate this change in Chemical State are called nitrifying bacteria. The 
reverse process of forming reduced nitrogen compounds from nitrate occurs 
mostly in anoxic sediments and is called denitrification; these changes are 
carried out by denitrifying bacteria. The nitrogen cycle also involves 
nitrogen-fixation, in which dissolved nitrogen gas is converted to organic 
nitrogen compounds; this process can be carried out by only a few 
phytoplankton, notably some Cyanobacteria. Dissolved organic nitrogen 
(DON) and particulate organic nitrogen (PON) both serve as nutrients for 
bacterial growth. Bacteria break down proteins to amino acids and ammonia, 
and the latter is oxidized in the nitrification process. The eventual release of 
dissolved inorganic nitrogen (DIN) makes these forms available again for 
uptake by the phytoplankton. The various types of bacteria involved in this 
cycling can themselves serve as a direct source of food for some nano- and 

An important aspect of the marine nitrogen cycle concerns the source of the 
nitrogen used in primary production. Some fraction of the primary 
production is derived from nitrogen recycled from organic matter within the 
euphotic zone; another fraction is derived from new nitrogen which comes 
from sources outside the euphotic zone (see Figures 5.19 and 5.20). New 
nitrogen is primarily nitrate entering the euphotic zone from below the 

Figure 5.19 The nitrogen cycle in the euphotic zone of the sea. The diagram illustrates the 
recycling of nitrogen that takes place within the euphotic zone and above the nutricline, as 
well as the ¡nput of ‘new’ nitrogen upwelled from deeper water. Note the interrelationships 
between DIN (dissolved inorganic nitrogen), PON (particulate organic nitrogen), and DON 
(dissolved organic nitrogen). The nutricline is where there is a rapid change in the 
concentration of a nutrient with depth. 

atmospheric exchange 

DIN runoff 


tota! phytoplankton production 
(gC m^yr 1 ) 

total phytoplankton production 
(g C m ' 2 yr 1 ) 

Figure 5.21 (a) Phytoplankton new production 

as a function of total (regenerated + new) 
annual phytoplankton production in different 
marine environments ranging from oligotrophic 
to eutrophic. 

(b) Total carnivorous fish plus squid biomass 
production as a function of total phytoplankton 
production in marine environments ranging 
from oligotrophic to eutrophic. The blue line 
indicates the total amount of fish production 
that results from new production only; this is a 
sustainable yield because the nitrogen removed 
in the harvest will be replaced through 
upwelling of nitrate. 


N 2 other N 
fixation inputs 



Figure 5.20 A comparison of production generated by recycled nitrogen and by new 
nitrogen. Regenerated production results only from the reduced nitrogen supplied within the 
euphotic zone by the excretion of organisms. New production depends on nitrogen supplied 
from outside the euphotic zone, of which the dominant source is the nitrate moving upward 
from below the nutricline. In a steady State, upwelled nitrogen is balanced by the downward 
sinking of nitrogen bound in sedimenting particles. On an annual time scale, new production 
in the open ocean is believed to be roughly one-third to one-half of regenerated production, 
but the actual valué may vary considerably from these figures over short temporal and 
spatial scales. P.Q., photosynthetic quotient (defined in the text). 







recycled nitrogen 
(NH 4 + etc.) 

upwelled , V 

N q - sedimentation 

3 of organic -N 

nutricline by vertical mixing, but it also ineludes smaller amounts of nitrogen 
entering by N 2 -fixation and through river inflow and precipitation. Recycled 
nitrogen is primarily in the form of ammonia and urea. This comparison of 
regenerated and new nitrogen (and of regenerated and new production, see 
Figure 5.20) is important because only the continual input of new nitrogen 
can determine the total capacity of the ocean to produce a sustainable fish 
harvest (remember that removing fish from the ocean also removes nitrogen). 
It is also only the new nitrogen that can help to take up the excess CO 2 that 
is believed to be entering the ocean from hurtian activities; in this case, 
increased production of phytoplankton removes more carbón dioxide. 

In oligotrophic areas of the oceans (e.g. the large convergent gyres discussed 
in Section 3.5.1), there is little upward movement of water from below the 
euphotic zone and thus the amount of new nitrogen is very small. In 
upwelling regions, however, the amount of new nitrogen is very large. The 
ratio of new production to total (new + regenerated) production is referred 
to as the/-ratio. This is shown in Figure 5.21a where it is related to waters 
of different nutrient concentrations. The valué of the /-ratio is probably 0.1 
or less in oligotrophic waters, but may be as high as 0.8 in upwelling zones. 
An annual average for the whole ocean is estimated to be about 0.3 to 0.5. 
About one-third of the global pelagic primary production takes place in areas 
where new nitrogen is entering the euphotic zone; these Coastal or upwelling 
areas represent only about 11% of the ocean surface (see Table 5.1). 
Elsewhere, primary production depends predominantly on nitrogen that is 
recycled within the euphotic zone. 


QUESTION 5.14 (a) If the annual primary production of a marine area is 

300 g C m _: and 30 c k of that production is driven by regenerated nitrogen, 
what is the valué of the /-ratio? (b) Where would you expect to find water 
with this amount of production and with this /-ratio? (Refer to Section 3.6.) 

Figure 5.21b relates the production of carnivorous fish plus squid to total 
primary production and to new production. The total amount of fish and 
squid produced results from both regenerated and new production. 

Harvesting that fraction of fish produced from recycled nitrogen will lead to 
a loss of nitrogen from the System and thus a decrease in production, but 
removing fish produced from new nitrogen sources will result in a 
sustainable harvest because the nitrogen will be replenished. Thus a larger 
fraction of the total fish production can be removed from eutrophic waters 
than from oligotrophic waters without depleting nitrogen in the surface 
layers. Figure 5.21b shows that a total fish production of 2 g wet weight 
m~ 2 yr _l in oligotrophic waters with an /-ratio of about 0.1 will give a 
sustainable annual harvest of 0.2 g wet weight m“ 2 yr' 1 (from 0.1 of 2 g). 

In comparison, 20 g wet weight m' 2 yr -1 of total fish production in 
eutrophic waters with an /-ratio of 0.8 provides a sustainable harvest of 
16 g m -2 yr _l , or an 80-fold increase in the sustainable harvest compared 
with only a 10-fold increase in total fish production. 

QUESTION 5.15 Why does the amount of fish available for a sustainable 
harvest increase exponentially in going from oligotrophic to eutrophic waters 
in Figure 5.21? The answers should be apparent in Figures 5.21a and 5.21b. 

In Figure 5.20, the photosynthetic quotient (PQ = moles of O 2 produced by 
the phytoplankton divided by the moles of CO 2 taken up) is used to diagnose 
the difference in the production processes involving the two types of 
nitrogen. Note that regenerated production based on recycled nitrogen has a 
lower PQ (% 1.2) than new production (PQ ~ 1.8) that is based on nitrogen 
entering from the atmosphere and rivers and, especially, on upwelled nitrate. 
The reason for this is summarized in Figure 5.22, where different valúes of 
PQ are obtained depending on which type of nitrogen is being utilized and 
which Chemical pathway is being followed in photosynthesis. If only 
carbohydrate material were being formed, there would be a stoichiometric 
yield of one mole of oxygen produced for each mole of carbón dioxide used. 
However, lipid material also is formed during photosynthesis, and because 
lipids are more highly reduced than carbohydrates, additional oxygen will be 
released and the PQ will approximate 1.2 instead of 1.0. If large amounts of 
nitrate (a new nitrogen source) are taken up and reduced in the process of 
forming proteins, even more oxygen is liberated relative to the CO 2 utilized, 
and thus the PQ will increase to about 1.8. If ammonia (a recycled nitrogen 
form) is used as a nitrogen source for protein manufacture by the 
phytoplankton, then oxygen is required in the process and the PQ would be 
1.0 or less. Thus rapidly dividing phytoplankton populations, using nitrate, 
will have relatively high PQ valúes. Lower PQ valúes indicate that reduced 
States of nitrogen, like recycled ammonia, are being used in photosynthesis. 
Because it is difficult to make accurate measurements of the PQ or, 
altematively, of the rate and amount of vertical nitrate transport, the /-ratio 
remains a matter of much discussion and debate among scientists. 



+ 0 2 PQ* 12 


(-CH P 0-J 

carbohy tírale 

+ 0-, P.Q. s 10 

N0 3 - 

(NH 2 ) +0 2 P.Q = 1.8 




NH 4 * 


+ H 2 0 PQ. = 0,8 

Figure 5.22 The relationship between photosynthetic pathways and the photosynthetic quotiení 
(P.Q.). The basic equation for photosynthesis is shown in the middle; in this case, only 
carbohydrate is being produced and the P.Q. would be 1.0 (1 mole of 0 2 is produced for each 
mole of C0 2 utilized). Note that the valué of P.Q. varíes according to the type of nitrogen source 
being utilized by the phytoplankton to form proteins. (See text for further details.) 

5.5.2 CARBON 

Carbón is another element that is essential for life, but unlike nitrogen, 
carbón is never present in the sea in limiting quantities. However, the carbón 
cycle (shown in Figure 5.23) has some special properties that involve both 
physical and biological processes. 

Carbón dioxide enters the ocean from the atmosphere because it is highly 
soluble in water. If the concentration of C0 2 in seawater depended entirely 
on the partial pressure of C0 2 in the atmosphere (0.3 mi l -1 ), on the relative 
concentrations of C0 2 in water and air, and on the temperature and salinity 
of the water, then the amount of C0 2 in seawater would be very low. In the 
sea, however, free dissolved C0 2 combines with water and ionizes to form 
bicarbonate and carbonate ions, as shown below. 

mineralization; and 
dissolution of CaC0 3 

C0 2 + H 2 0 H 2 C0 3 

- HC0 3 + H + 





photosynthesis; and 
formation of CaC0 3 

C0 3 - + H + 




These ions are bound forms of carbón dioxide, and they (especially 
bicarbonate) represent by far the greatest proportion of dissolved carbón 
dioxide in seawater. On average, there are about 45 mi total CO 2 1 _1 of 
seawater, but because of the equilibrium Chemical reactions shown above, 
nearly all of this occurs as bound bicarbonate and carbonate ions which thus 
act as a reservoir of free CO 2 . The amount of dissolved CO 2 occurring as 
gas in seawater is about 0.23 mi l -1 . When free CO 2 is removed by 
photosynthesis, the reaction shifts to the left and the bound ionic forms 
release more free CO 2 ; so even when there is a lot of photosynthesis, carbón 
dioxide is never a limiting factor to plant production. Conversely, when CO 2 
is released by the respiration of plants, bacteria and animáis, more 
bicarbonate and carbonate ions are produced. 

Note that hydrogen ions are liberated in the general Chemical reactions 
shown above. This means that the pH of seawater is largely regulated by the 
concentrations of bicarbonate and carbonate, and the pH is usually 8 ± 0.5. 
When CO 2 is added to seawater due to mineralization processes and 
respiration, the number of hydrogen ions increases and the pH goes down 
(the solution becomes more acidic). If CO 2 is removed from the water by 
photosynthesis, the reverse happens and the pH is elevated. Thus seawater 
acts as a buffered solution. 

Some marine organisms combine calcium with carbonate ions in the process 
of calciflcation to manufacture calcareous skeletal material. The calcium 
carbonate (CaCC> 3 ) may either be in the form of calcite or aragonite, the 
latter being a more soluble form. After death, this skeletal material sinks and 
is either dissolved, in which case CO 2 is again released into the water, or it 
becomes buried in sediments, in which case the bound C0 2 is removed from 
the carbón cycle. 

QUESTION 5.18 Which marine organisms (planktonie, nektonie and benthic 
speeies) incorpórate carbón dioxide into carbonate skelelons and thus 
influence the carbón cycle? íRefer to Sections 3.1 and 4.2 for pan of the 

The simplified carbón cycle shown in Figure 5.23 summarizes the processes 
discussed above. In general, CO 2 is converted from inorganic to organic 
carbón by the photosynthesis of the phytoplankton. This is then consumed by 
the higher trophic levels, and some CO 2 is recycled as inorganic bicarbonate 
while some may be lost from the ocean surface in gaseous form. Carbón 
dioxide is absorbed at the ocean surface and is produced in the water column 
by respiration and mineralization processes. It is believed that more carbón 
dioxide is being absorbed by the oceans than is being lost to the atmosphere. 

The total amount of soluble carbón dioxide (bicarbonate and carbonate ions 
plus dissolved CO 2 ) in the world’s oceans is estimated to be about 38 x 10 12 
tonnes. This is about fifty times more than the total carbón dioxide in the 
atmosphere. The burning of fossil fuels is increasing the total carbón dioxide 
in the atmosphere at a rate of about 0.2% per year, and it is very important 
to know if this increase can be absorbed by the oceans, or if it will continué 
to accumulate in the atmosphere where it may contribute to global warming 
by the process often referred to as the ‘greenhouse effect’. 

The importance of the ocean’s biology to the carbón cycle and to the 
balance of CO 2 in the atmosphere is threefold. Firstly, the amount of CO 2 



* cg 3 - 



free dissoived 
/ CO, 

bicarbo nale 


free dissolved 

' CCW 




- ; 





d resolved 
^ detritus 



s CaCO, „ 





Livjng material 

Norhlfving material 

Figure 5.23 The basic scheme of the carbón fixed by the food chain depends on how much new nitrate enters the 
cyde- euphotic zone to support photosynthesis (see Section 5.5.1). Secondly, the 

amount of carbón which can be permanently lost to the sediments depends 
on deep-water chemistry, ecology and sedimentation processes, and 
particularly on the bacterial loop (Section 5.2.1) which recycles dissolved 
and particulate organic carbón. Thirdly, the amount of carbón dioxide taken 
up in the carbonate skeletons of marine organisms has been, over geological 
time, the largest mechanism for absorbing CO 2 . At present, it is estimated 
that about 50 x 10 15 tonnes of CO 2 occurs as limestone, 12 x 10 15 tonnes in 
organic sediments, and 38 x 10 12 tonnes as dissolved inorganic carbonate. 
Determining the amount of carbón that is transferred along the various 
pathways in Figure 5.23 is a difficult problem in both geochemistry and 
biology, but a necessary exercise if we are to solve the global carbón budget. 

CUESTION 5.17 Carbón dioxide is essential to the process of photosynthesis. 
Should it be considered as a nutriera, like nitrate for example. that can limit 
the rate of phytoplankton production in the oceans? 


1 Food chains are ways of describing the linear passage of energy and 
organic materials contained in food from the first trophic level of primary 
producers, through the consumer levels of herbivores and camivores, to the 
top-level predators. There is an energy loss with each transfer between 
trophic levels because of metabolic demands and conversión of Chemical 
energy to heat. However, Chemical elements that are incorporated in food are 


recycled through the decomposition of organic materials; this process 
releases dissolved inorganic compounds that can once again be taken up by 
phytoplankton and converted to organic compounds during photosynthesis. 

2 Despite great differences in size between phytoplankton and consumers 
in higher trophic levels, the differences in generation times (hours to many 
years) among these organisms result in very similar biomass valúes in each 
trophic level of marine food chains. 

3 Estimates of secondary production in different marine localities can be 
made by using the expression 1 ) = P\E n . This equation combines 
quantitative valúes for primary productivity in an area with the number of 
trophic levels in the food chain, and with the ecological efficiency at which 
energy is transferred from one trophic level to another. 

4 The number of trophic levels in a food chain is inversely correlated with 
the predominant size of the phytoplankton. Food chains in nutrient-rich 
upwelling areas are characterized by having large chain-forming diatoms, 
high primary productivity, few trophic levels, and a high biomass of fish or 
marine mammals. In the nutrient-poor open ocean, the primary producers are 
nanoplanktonic autotrophic flagellates with relatively low productivity; this 
leads to long food chains and, because of increased energy loss in longer 
food chains, there is a relatively lower biomass of top-level predators. 

5 Food webs are more realistic, but more complex, depictions of energy 
flow through interacting species. They are a means of recognizing that many 
marine species compete for the same food Ítems, that many animáis change 
diets during life, that some organisms feed primarily on detritus, and that 
cannibalism is common in the sea. Such relationships may affect the amount 
of energy that is available for top-level predators; for example, competition 
for food between ctenophores and larval fish, or ctenophore predation on fish 
eggs and fish larvae, may significantly lower fish stocks that are harvested 

6 Bacteria and planktonic protozoans internet in a microbial loop that is 
coupled with the elassie phytoplankton-zooplankton-fish food chain. In this 
subsystem, bacteria decompose particulate and dissolved detritus; the 
resulting bacterial production is consumed by protozoans and by some larger 
zooplankton, such as invertebrate larvae and appendicularians. Thus bacteria 
regenérate dissolved nutrients for subsequent utilization by phytoplankton, 
they themselves form a source of food for planktonic bactivorous species, 
and the bacterial production is transferred to higher trophic levels by the 
intermediary links of protozoans which are fed on by larger plankton. 

7 Major differences exist between marine and terrestrial food webs. The 
majority of marine primary production is carried out by fast-growing 
microscopic phytoplankton, most of which is consumed and assimilated by 
herbivores. In contrast, most terrestrial vegetation is large, slower growing, 
and contains much indigestible structural material. Only 15% or less of the 
total terrestrial plant production is eaten, and only a fraction of this is 
digestible and assimilated into herbivore production. Further, the dominant 
marine animáis are poikilothermic, with lower metabolic energy demands 
than the homoiothermic birds and mammals that live in terrestrial habitats. 
This difference in energy utilization, coupled with the fact that most marine 
primary production is eaten, means that energy is transferred with greater 
efficiency through marine food chains, and that there is a much higher 
secondary production in the sea compared with that on land. 


8 Whereas it is possible to obtain fairly accurate estimates of primary 
production through various techniques, and ñsh catch statistics provide 
mínimum valúes for energy output from marine food webs, it is much more 
difficult to quantify secondary production in the intermedíate trophic levels 
occupied by zooplankton and smaller nekton. Although some techniques 
have been applied to measure secondary production from field data, these are 
often impracticable because of the vast geographic areas under consideration 
and the continual movement of the water and resident organisms. Many 
researchers have therefore resorted to different experimental options. These 
inelude laboratory-scale experiments, which attempt to quantify each aspect 
of energy partítioning in a species; the use of controlled ecosystem 
experiments, which are carried out on a larger scale and attempt to study 
several interacting trophic levels at one time; and Computer model 
simulations, in which data from various sources are entered into 
mathematical models that attempt to simúlate natural processes. 

9 The term ‘mineralization’ describes the process whereby elements that 
have passed through food webs are reeyeled. Ecological studies are 
particularly concerned with the reeyeling rates of essential nutrients that may 
be present in limiting concentrations in the sea; these inelude nitrate, iron, 
phosphate and, occasionally, dissolved Silicon. Of these nutrients, nitrate is 
the one that is most often present in sufficiently low concentrations to limit 
plant growth. Nitrogen has a complex eyele in the sea because it occurs in 
many forms; nitrate is the dominant form most often utilized by 
phytoplankton, but ammonia, nitrite, and dissolved molecular nitrogen can 
also be used by some species. The physiological activities of organisms 
produce particulate and dissolved organic nitrogen in various Chemical 
species, and different types of bacteria medíate the conversions from one 
type of nitrogen compound to another. 

10 An important distinction is made between regenerated nitrogen 
(primarily ammonia and urea) that is reeyeled in the euphotic zone by 
pelagic organisms, and new nitrogen (primarily nitrate) that enters the 
euphotic zone from upward movement of deep water or, in smaller amounts, 
from river inflow and precipitation. The amount of new nitrogen relative to 
regenerated nitrogen (the /-ratio) is high in upwelling regions and low in 
oligotrophic areas. Where production is based primarily on new nitrogen 
(nitrate), the photosynthetic quotient (PQ) is high, indicating the formation of 
proteins from nitrate with release of oxygen. If photosynthetic production is 
based on regenerated nitrogen forms (i.e. ammonia), the PQ is low as oxygen 
is required in the reaction. It is the continual input of new nitrogen that can 
elevate primary production levels and ultimately sustainable fish harvests. 

11 Carbón is essential for Ufe, and is never present in limiting quantities in 
the sea. This is because dissolved carbón dioxide enters into equilibrium 
reactions with bicarbonate and carbonate ions. As more CO 2 enters the sea 
from the atmosphere, or as the result of physiological activities (primarily 
respiration), more bicarbonate and carbonate ions are formed, thus increasing 
the amount of CO 2 which can continué to enter the sea. Conversely, when 
there is a biological demand for dissolved CO 2 , the Chemical reactions are 
reversed and CO 2 is released from its bound ionic States. As the quantities of 
CO 2 entering the atmosphere from human activities increase, it becomes 
increasingly important to determine how much can be absorbed by the seas 
and how much will accumulate in the atmosphere where it may contribute to 
global warming. 

Noxv try the following questions to consolídate your understanding ofthis 

QUESTIOH 5.18 Assume that in an open ocean región the priman production 
is 1000 g wet weight m 2 yr _l , and there is a 20% transfer efficiencv 
beiween the primary producers and primary consumera, and 10% efííciency 
between all successive trophic levels. What is ihe máximum amount of the 
priman' production that can be eonverted to íish in the highest trophic level? 

QUESTI0N 5*19 Considering the relatíve age when different types of 
phytoplankton and different top predators ürst appeared in the geologic 
record (see Appendíx I), what can you deduce about the evolution of the 
types of food chains shown in Figure 5,3? 

0UESTI0N 5.20 What might happen to a regional ítshery if mosi of the 
piscivorous Iish (e.g* tuna) were removed, leavíng mosUy planktivorous íish 
(e.g. sardines) to harvest? Consider the effects on the regional food Chain in 
terms of energy transfer and relatíve numbers of organisms in different 
trophic levels, 

QUESTI0N 5,21 In one región of the North Atlantic, the production of sand 
eels, which feed on zooplankton, has beert determined from Iish catch data to 
be 0*5 tonnes wet weight per hectare per year, and the phytoplankton 
production in the same area is 200 g C m : yr E * Assuming that carbón 
makes up 50% of the drv weight of Iish and that the dry weight is 20% of 
the wet weight. what is the uverage ecological efííciency oí this system? 
(Note: 1 hectare — 10000 nr.) 

QUESTIOH 5*22 In each of the following situatiom, would it be best to use a 
laboratory experiment. un enclosed experimental ecosvstem, or a Computer 
model simulation.? 

(a) to study the effeet of pesticide runoff from agricultura! land on neritic 

(b) lo examíne the potential environmental impaets of Hamming an estuary: 

(c) to investígate physiological properties of plañís or anintals, 

QUESTIOH 5,23 The chaetognath Sagina elegtms consumes 5 mg of copepods 
per day and produces 0*75 mg of faecal material per day. What is the 
assimilatkin efficiency oí this camivore? 

QUESTIOH 5.24 Would it be praeüeal to add nutrients to a snía II hay in order 
to enhance oyster cultures? 

QUESTIOH 5*25 Assuming that the amount of new nitrogen remains constant. 
what would happen to the productivity of an ocean area if many of the 
residem Iish were removed by a commercial fishery? 



Fish make up the largest fraction of the nekton, but large crustaceans, squid 
and related cephalopods, sea snakes, marine turtles, and marine mammals 
can be important nektonic species in certain areas. Large nektonic animáis 
and seabirds can have profound influences on marine communities in terms 
of predation. As well, many of these animáis figure importantly in 
commercial harvests as sources of food, fur, or other commodities, or they 
have done so in the past. Fish dominate the present marine catch, and squid 
are being taken in increasing numbers; the catch of marine mammals and 
marine turtles is declining through public pressure for conservation of many 
of their species. 


Although there are pelagic swimming crabs and shrimp that fall in the 
category of nekton, relatively little is known about the biology of the 
different species and few of them are abundant enough to be of commercial 
interest. Ninety-five per cent of commercially harvested crustaceans are 
demersal species that are caught in benthic trawls. However, considerable 
attention has been given to euphausiids (see Section 4.2) as an exploitable 
resource, as one species in particular is exceedingly abundant. 

With the decline in whale numbers and the consequent cessation of most 
commercial whaling, Euphausia superba , the Antarctic krill and predominant 
food of baleen whales (see Figure 5.4), became an alternad ve fisheries target. 
Although rarely used for human food, these large (5-6 cm long) euphausiids 
can be dried and processed into feed for livestock, poultry, and farmed fish. 
Russian and Japanese fieets began harvesting krill in the 1960s, and the peak 
catch was 446000 tonnes in 1986. Economic considerations closed the 
Russian harvest, and in 1994 the commercial harvest was only about 100000 
tonnes, taken by fieets from Japan and Chile. This is an insignificant fraction 
of the amount consumed by natural predators, estimated at about 470 million 
tonnes annually (see Table 5.2). The potential krill harvest has been 
estimated to be at least 25-30 million tonnes a year, or about one-third of 
the present world fish catch. However, the economic costs of fishing in the 
remóte Antarctic are relatively high and, although krill form vast swarms, 
the congregations are widely scattered and sometimes located at depths of 
150-200 m. Nevertheless, once a swarm has been located by echo-sounding, 
a single net haul from a large fishing vessel may commonly catch 10 tonnes 
of krill. The ecological consequences of removing vast numbers of krill on 
the balance of populations in the Antarctic ecosystem (including recovery of 
whale numbers) are not clear but, because krill are central to the Antarctic 
food web, there is reason for caudon in expanding this harvest. 

There is also a commercial harvest of a smaller euphausiid, Euphausia 
pacifica , along the northeastern Japanese coast. This particular fishery 
depends upon the unique fact that, in this area during the spring, the 
euphausiids form surface schools and are therefore easily accessible. The 
harvest of about 60000 tonnes per year is mainly processed for use as feed 


in fish farms. The euphausiids provide a rich source of both protein and 
vitamin A; the latter is believed to enhance the texture and pigment of the 
flesh of the farmed fish. 


Squid (Figure 6.5g), cuttlefish, and octopods (e.g. Octopus) are the 
molluscan members that make up the Class Cephalopoda. Squid constitute 
approximately 70% of the present catch of cephalopods, and estimates 
indicate that the harvest could be increased appreciably. The potential world 
catch of squid is estimated conservatively to be 10 million tonnes annually. 
Despite their abundance, surprisingly little is known of the biology and 
ecology of many species. 

Squid range in size from a few centimetres up to the legendary deep-sea 
giant squid {Architeuthis) that exceeds 20 m in length (with outstretched 
tentacles) and 270 kg in weight, and thus attains the status of being the 
largest of all living invertebrates. All squid swim by propulsión, ejecting jets 
of water from their siphon, and these streamlined cephalopods rival fish in 
swimming ability and manoeuverability. Some of the larger squid species are 
capable of speeds of about lOms" 1 . They are also rivals with some fish for 
food, as squid typically eat 15-20% of their body weight per day, taking a 
variety of zooplankton as well as smaller fish and other squid as prey. 

Many of the very abundant squid species are extensively fished and form a 
major source of human food in some countries. In 1981, the Japanese began 
using driftnet fishing to harvest squid in the Pacific Ocean. Driftnets are 
panels of monofilament webbing measuring 8-10 m in width and up to 
50 km in length; mesh size is usually 90-120 mm. At night, the nets were 
placed vertically in the open ocean and allowed to drift with the winds and 
currents for about 8 hours in order to snare squid and fish. By 1989, Japan, 
Korea, and Taiwan were deploying about 800 driftnet vessels in the Pacific 
to harvest 300000 tonnes of squid annually, and an estimated 200 other 
vessels were operating in the Atlantic and Indian oceans. In addition to 
squid, these almost invisible nets indiscriminately captured a large number of 
other species. In the North Pacific, salmón were the most common (and 
illegal) by-catch; in the South Pacific, albacore tuna were a valuable catch 
totalling 60000 tonnes in 1988. By 1989, there was growing concern about 
the numbers of animáis being captured incidentally by this method. In 
addition to nonselective capture of other fish species (including sharks) and 
turtles, it was estimated that between 750 000 and 1 000000 seabirds and 
20000 to 40 000 marine mammals were being killed annually in the nets 
used in the Pacific alone. There were no estimates of the numbers of large 
zooplankton, such as salp chains and jellyfish, that may also have been 
destroyed by the nets. The seriousness of removing such vast numbers of 
animáis from the oceans led the United Nations General Assembly to accept 
a resolution calling for an intemational moratorium on all large-scale, high 
seas, driftnet fisheries in 1993. Although some driftnets continué to be used, 
the problem has abated. 

QUESTIQN 6.1 Asstiming a nightly deploymeni of 800 driftnets during the 
I980s, how many küometres of net were set out eaeh night by the squid 
fleets in the Pacific Ocean? 


Squid can be harvested by more selective fishing methods. In Japan, about 
500 000 tonnes of Todarodes pacificus are caught each year using a 
technique that captures only squid. This particular squid undertakes an 
extensive annual migration of about 4000 km, moving from the spawning 
grounds in the northern part of the East China Sea (about 32°N) to the 
vicinity of the Kurile Islands (45°N) before returning. 


There are comparatively few reptiles that have adapted to a marine life. The 
best known are the eight species of marine turtles, but there are more than 
six times as many species of sea snakes, and there is one marine lizard, a 
large seaweed-eating iguana of the Galápagos Islands. Several crocodiles live 
in Coastal waters, the largest being the infamous Australian species 
Crocodylus porosus. 

Marine turtles usually are found in tropical waters, but some migrate or are 
carried by currents to températe shores. Some turtles feed on jellyfish or fish 
in the open ocean, others (the green turtle) on shallow-water seagrasses, but 
all undertake long migrations to return to land in order to lay their eggs at 
specific nesting sites on sandy shores. High mortality is infíicted on the eggs, 
which are eaten by natural predators and are also prized by humans. Newly 
hatched young also have high mortality rates as they are preyed upon by 
birds and crabs during their scramble for the sea, and by predatory fish 
during their early life in the water. Adults have been hunted nearly to 
extinction for their meat and decorative shells. All sea turtles are now 
considered to be threatened or endangered species and conservation methods 
are in forcé in many countries, including bans on capture and importation of 
turtle producís. Efforts are being made in several areas of the Indo-Pacific 
and Caribbean to gather eggs and keep these until hatching, at which time 
the young are released directly into the sea. It remains to be seen whether 
these protective measures will restore population numbers. 

Sea snakes breathe air by means of nostrils and lungs, but they are truly 
marine animáis that inhabit Coastal estuaries, coral reefs, or open tropical 
water. Most of the approximately 60 species remain at sea to bear their live 
young. They school in large numbers and feed on small fish or squid which 
they kill with venom injected by fangs. Sea snakes are extremely poisonous 
and, although not all are aggressive, they have caused human deaths. They 
themselves have few predators except for sea eagles, sharks, and saltwater 
crocodiles. Sea snakes are presently restricted to warm waters of the Indian 
and Pacific oceans, but there has been concern that a new sea-level canal 
through the Isthmus of Panama would allow their passage into the warm 
waters of the Caribbean and Atlantic. They are presently excluded by a 
freshwater barrier in the canal. 


There are three orders of mammals that have evolved from different 
terrestrial ancestors and independently adapted to life in the sea. These three 
orders inelude respectively: the whales, dolphins and porpoises: the seáis, 
sea lions, and walruses; and the dugongs, manatees, and sea cows. All share 


the mammalian characteristics of being warm-blooded (homoiothermic) and 
nursing their young, and they all rely on breathing air. 

The order Cetácea comprises the 76 or so species of marine mammals 
known as whales, porpoises, and dolphins. The ancestors of this group were 
land animáis that entered the sea about 55 million years ago. The largest of 
these marine mammals are the baleen whales (Figure 6.1); these inelude the 
biggest animáis that have ever lived, the blue whales, which can attain a 
length of 31 m. 

Baleen whales form a sepárate suborder (Mysticeti) of about ten species. 
Like the largest of the sharks, most of these immense whales feed primarily 
on zooplankton that they strain through specialized homy plates called 
baleen or whalebone. The brush-like baleen hangs down from the roof of 
the mouth on both sides, and food that collects on the baleen is periodically 
removed by the tongue of the whale. The humpback and finback whales also 

J_i 1 i_I_i- - i 

Figure 6.1 Relative sizes of baleen and 






15 18 





toothed whales. 



are capable of capturing schools of relatively large fish, such as mackerel 
and herring, and the grey whale suction-feeds on benthic animáis. 

Some of the large baleen whales (e.g. greys, humpbacks) make extensive 
seasonal migrations, usually breeding in winter in tropical waters and 
moving poleward to feed in summer. Smaller cetaceans do not undertake 
long migrations, but move in response to changing food supplies or physical 

QUESTIQN 0.2 Oí what advamage is it to mígrating whales to have 
warm-water breeding nursery areas and high-latitude summer feed i ng sites? 

The suborder Odonticeti ineludes the other 66 species of cetaceans, all of 
them equipped with teeth and characterized by having a single blowhole 
instead of the two of baleen whales. The odontocetes inelude the remaining 
whales, dolphins, and porpoises (Figure 6.1). The toothed whales are 
formidable predators in the sea, taking squid or fish as prey or, in the case of 
some killer whales, even other whales or seáis and sea lions. Unlike some of 
the baleen whales, these animáis are not reliant on surface-living prey, and 
they may undertake dives to depths of several hundred metres. The sperm 
whale holds the record among marine mammals for deepest dives; it is 
believed to descend to over 2200 m in search of giant squid. Some 
odontocetes hunt prey by echolocation, in which they emit pulses of sound 
and monitor the returning echoes, and at least some species show 
cooperative behaviour in herding and capturing their prey. 

Some scientists have suggested that the cetaceans as a whole, and possibly 
the sperm whales alone, consume a greater quantity of prey than is taken by 
the entire world commercial fishery. For example, between 1979 and 1982, 

18 species of cetaceans consumed between 46000 and 460000 tonnes of 
prey annually on Georges Bank, off the northeastem United States, 
compared with a commercial harvest in the same area of 112000-250000 
tonnes. In the Mediterranean, where squid figure importantly in the human 
diet, cetaceans are estimated to eat about 2.3 times more squid than are 
taken by humans. And before commercial whaling fleets decimated whale 
populations in the Antarctic, baleen whales may have taken about 190 x 10 6 
tonnes of krill annually, a figure that represents more than twice the total 
world catch of all marine species (see Section 6.7.1). Given these figures, it 
is not surprising that fishermen often regard cetaceans as competitors for fish 
and squid stocks. 

The Inuit have been hunting marine mammals since time immemorial, but 
the earliest recorded whaling began off northem Europe between A.D. 800 
and 1000 and whaling became a major commercial enterprise in the 1700s 
and 1800s. Whales were exploited primarily for their oil, which was used in 
lamps, and for whalebone (baleen) used to stiffen women’s apparel; whale 
meat was of secondary importance, except in Japan. The advent of increased 
mechanization, motorized high-speed ships, and explosive harpoons in the 
early 1900s resulted in rapid declines in whale populations and threatened 
extinction of some species. Even after the establishment of the International 
Whaling Commission (IWC) in 1946, the annual catches of whales 
continued to climb to about 65 000 during the 1960s. Pre-exploitation and 
present population estimates of 13 species are given in Table 6.1; of these 
species, nine have been listed as endangered or vulnerable species since 
1970. Given the low fecundity and slow development times of the great 


Table 6.1 Past and present population estimates of whales. All estimates are taken 
from the International Whaling Commission and most are highly speculative. 

Common ñame 

Scientific ñame 

Population estimates 



Baleen whales: 

Balaenoptera musculus 


<10 000* 


Balaenoptera physalus 




Balaenoptera borealis 




Balaenoptera edeni 




Balaenoptera acutorostrata 


725 000 


Balaena mysticetus 



Northern right 

Eubalaena glacialis 

No estimate 


Southern right 

Eubalaena australis 




Megaptera novaeangliae 

115 000 



Eschrichtius robustas 



Toothed whales: 

Physeter catodon 

2 400000 

1 950000* 


Monodon monoceros 

No estimate 

35 000 


Delphinapterus leucas 

No estimate 

50 000 

*Listed as an endangered or vulnerable species by the International Union for the 
Conservation of Nature or by the United States Government. 

whales, severely depleted populations may take decades to recover. Only one 
species, the minke whale, is known to have greatly increased in abundance. 
The relatively small minkes were never heavily exploited, and the Southern 
population might have increased due to the decline of larger baleen species 
with which it may compete for krill. In 1986, the IWC voted to establish an 
indefinite ban on commercial whaling in the hope of re-establishing 
endangered stocks. In 1994, the IWC designated 28 x 10 6 km 2 around 
Antárctica as a whale sanctuary, thus providing permanent protection for 
about 90% of the world’s whales. These measures do not, however, slow 
cetacean mortality due to other factors. The smaller dolphins and porpoises 
are captured incidentally by fishing gear; and cetaceans that reside in, or 
temporarily enter, Coastal waters are subject to increasing habitat destruction 
and pollution. There is no doubt that pollution threatens beluga whale 
populations in the St. Lawrence Estuary of eastem Cañada, and river 
dolphins in many other areas. 

A second order of marine mammals ineludes the seáis, sea lions, and 
walruses. These familiar animáis are known taxonomically as pinnipeds 
(order Pinnipedia), meaning Teather-footed’ to describe their four swimming 
flippers. In contrast to whales, these animáis spend part of their time on land 
or on ice floes, where they congrégate for breeding and resting. The 32 
species of pinnipeds are found in all the seas of the world, and there is one 
freshwater species in Lake Baikal, but the majority of species and the largest 
populations are found in the coid waters of the Arctic and Antarctic. Most 
feed primarily on fish or squid, but walruses also use their tusks to dig 
molluscs and other benthic animáis from the sea bottom. Pinnipeds typically 
live and travel in herds, and some may undertake long migrations at sea. 

Although seáis and sea lions have been heavily exploited in the past for their 
fur and oil, and walruses for their ivory, hunting pressure has lessened for 
most of the species. However, the Caribbean monk seal is now believed 
extinct, and the Hawaiian and Mediterranean monk seáis remain endangered. 


Manatees and dugongs belong to the mammalian order Sirenia. They are the 
only herbivorous aquatic mammals, and they rely on larger plants, not algae, 
for nourishment. Their food requirements restrict them to living in shallow 
Coastal waters, estuaries, and rivers. All four species of this order reside in 
warm waters and do not come on to land. Manatees and dugongs are thought 
to have been highly social animáis, as oíd records report huge congregations 
of these animáis before hunting decimated their numbers. The few remaining 
individuáis have tended to become solitary or form only small family 
groups. The sirenians have been particularly vulnerable to hunting pressure 
because of their inshore habitats and their slow and placid behaviour, and 
they are prized for their meat, oil and hides in many cultures. At one time, 
dugongs had a widespread distribution which included Atlantic waters; 
today, they are restricted to the Indian and Pacific oceans. All three species 
of manatees are found only in tropical Atlantic waters. 

A fifth species of sirenian, Steller’s sea cow (Hydrodamalis gigas), became 
extinct within historical times. The existence of these huge animáis was 
documented only once, by Georg Wilhelm Steller who was acting as a 
physician and naturalist on an expedition sailing in the far North Pacific 
under Commodore Vitus Bering. The St. Peter was shipwrecked in 1741 on 
a small island near the western end of the Aleudan chain. Although Captain 
Bering and many of his crew died from sickness in the first days after 
landing, the remaining survivors found adequate food in the resident otters 
and seáis. They also discovered sea cows in nearby waters and were 
eventually able to capture these for a source of meat and oil. The animáis 
were described as attaining a length of up to 10 m and a weight of 10 tons. 
They fed on kelp and other large seaweeds, chewing the plant blades with 
horny plates covering the palate and jaws. They formed small herds, and 
were slow-moving and of a passive temperament. The shipwreck survivors 
brought word of their discoveries when they returned to Russia in 1742, and 
future whaling and fur-hunting expeditions to the Bering Sea began to rely 
on sea cows as a source of food during the winter. Although 2000 Steller sea 
cows were estimated to live in the región, the last was killed in 1768, only 
27 years after the discovery of this population. Fossil evidence indicates that, 
as recently as 20 000 years ago, this sea cow inhabited Coastal areas as far 
south as California; although the cause of its extinction in these areas is a 
matter of conjecture, the animal was probably easy prey for pre-historic as 
well as modern humans. 


Like the marine reptiles and mammals, seabirds have evolved from land 
species that readapted to life in the sea. There are now approximately 
260-285 species of seabirds, depending on how the term is defined; these 
species represent roughly 3% of the world’s birds. Those birds that are most 
highly adapted to the marine environment inelude the auks, albatrosses, 
petrels, penguins, and gannets, all of which have few representatives on land 
or freshwater and spend 50-90% of their lives at sea. At the other extreme 
are the shorebirds, like sandpipers and plovers, that depend upon marine 
food sources but are incapable of swimming. 

The many species of oceanic birds have developed diverse methods of 
feeding and take different types of prey (Figure 6.2); this is reflected in 
species differences in structure of the bilí and wings, Some species 

storm petrel prion 







r* phalarope 

Figure 6.2 Seabird feeding methods. In most cases, the examples given represent only one 
of several types of birds that feed in the depicted manner. 

(skimmers, gulls, petrels) skim neuston from the immediate surface layers of 
the ocean, others (pelicans, tems, gannets) plunge deeper into the water to 
seize zooplankton, squid, or fish. Penguins, cormorants, murres, and puffins 
actively pursue their prey underwater, using their wings or feet for 
swimming. Although emperor penguins may dive to depths of more than 
250 m, the majority of seabirds are essentially dependent on the uppermost 
layers of the sea for their food. The impact of seabird predation on oceanic 
surface life has often been neglected, but it may be considerable. 

Although seabirds are found world-wide, the largest colonies are located 
adjacent to highly productive ocean areas where food is plentiful and 
concentrated. Millions of penguins (six species) are present in the Antarctic, 
where they depend for food on the vast numbers of krill or the abundant fish 
and squid that occur in this productive ocean. Equally large numbers of birds 
form island colonies in the upwelling Coastal regions off western South 
America (see Figure 6.10). At sea, birds frequently form feeding 
aggregations along oceanic fronts which, like upwelling regions, have 
relatively high biological productivity (see Section 3.5). Far fewer birds are 
present in low-productivity tropical regions. Seasonal changes in the marine 
environment can be reflected in the distribution of birds, and some species 
undertake long annual migrations in response to seasonal food availability 
and suitable weather for breeding. 


Shorebirds known as red knots undertake one of the longest migrations that 
is linked to exploiting seasonally available marine food resources. The 
American subspecies (Calidñs canutas rufa ) spends the austral summer at 
the Southern tip of Argentina, feeding primarily on young mussels in 
intertidal areas. It migrates northward in March, stopping to feed on clams, 
mussels, and worms along the South American coast. By late May, over 
100000 birds flock in Delaware Bay, along the eastem seaboard of the 
United States. Their arrival is timed to coincide with the breeding cycle of 
horseshoe crabs (Figure 7.7g) that come ashore in thousands to lay millions 
of eggs. Their eggs provide a high energy food source for the birds, and this 
fuels their remaining flight to islands in the Canadian Arctic. Each female 
lays four eggs shortly after arrival at the breeding grounds; these weigh 
about 75 g, or more than 50% of the female’s weight. The period spent in 
the Arctic coincides with peak abundances of insect and aquatic life, and the 
return southward migration in July and August occurs when local marine 
invertebrate populations are highest along the Atlantic coast of America. The 
birds increase their weight by about 40% within the few weeks spent feeding 
intertidally, then resume their migration to South America. (See Section 8.5 
for additional information on the impacts of shorebirds feeding on benthic 

Seabirds exhibit natural fluctuations in population densities which can be 
caused by climate change and subsequent fluctuations in prey availability. 
This has been documented, for example, in the bird colonies found on rocky 
uninhabited islands off the coast of Perú (see Section 6.7.2). Over 
evolutionary time, birds evolve adaptations that permit them to exist within a 
range of natural climatic variability. Unfortunately, seabirds are increasingly 
faced with new sources of mortality inflicted by human activities, and these 
occur with a tempo that preeludes slow evolutionary adaptation. 

All seabirds depend on nesting sites on land for breeding, and it is here that 
they encounter their greatest risks. Seabirds on land are exceptionally 
vulnerable to predation as it is difficult for them to defend themselves, and 
their eggs and young, against land mammals and snakes. Many species nest 
on inaccessible rock islands that are naturally free of mammalian predators. 
However, there are numerous examples of the delibérate or accidental 
introduction of predators, such as cats, rats, and pigs, that have resulted in 
disturbance or destruction of bird colonies. 

CUESTION 6.3 What are the predators of Antarctic species of penguins? 

(Reler to Figure 5.4.) 

Some seabirds have been exploited for their feathers, meat, eggs, or body 
oil. Only one, however, has been exterminated within historical time. This 
was the great auk (Pinguinus impennis) (Figure 6.3), which lived only on 
isolated islands in the North Atlantic. This Northern Hemisphere bird was 
the ecological equivalent of the Southern penguins; like penguins, it was 
large (to 1 m tall), flightless, and dived to pursue its marine prey. The giant 
auk was discovered in 1534, when there were several hundred thousand 
birds, but it was hunted to extinction within 300 years. At first hunted for 
meat by local fishermen, these seabirds were later harvested commercially 
for their feathers and oil. The last great auk was killed on Funk Island on 
June 3, 1844. Thousands of giant auk carcasses discarded on the rocky 
island provided fertilizer to nourish grasses, and today the island is a 
sanctuary for puffins and murres. 


Figure 6.3 The giant auk as painted by John James Audubon in about 1835, nine years 
before this flightless bird was hunted to extinction. 

Increasingly, seabirds are encountering greater mortality from Coastal 
pollution. In all oil spills, seabirds are usually the most obvious victims, and 
the effects of oil pollution are too well known to document here. Although 
most spills are localized, the mortality may be high; for example, an 
estimated 500000 birds perished from the effects of the oil spill of the 
Exxon Valdez in Alaska. Pesticide residues, working their way up the food 
web and accumulating in the bodies of seabirds, have caused thinning of egg 
shells and reduced hatching success in pelicans, ospreys, and other species. 
Other toxic Chemical pollutants which enter the sea and may affect birds 
inelude organochlorines, PCBs (polychlorinated biphenyls), and heavy 
metáis like mercury. Also of concern is the continuing loss of feeding and 
reproductive habitats through development of coastlines (see also Chapter 9). 

Increased fishing efforts have also affected seabird numbers. Large numbers 
of birds have been incidentally captured and drowned in driftnets in the 
North Atlantic and Pacific (see Section 6.2). Perhaps larger numbers are 
affected by reduction in food, through harvests of their prey. In Norway, for 
example, puffins have declined because of overfishing of the immature 
herring that are their main food. 

As is the case with many other animáis, seabirds are faced with new sources 
of mortality and an accelerated pace of change. Their past evolutionary 
experience, developed over 60 million years, will be of little valué in 
establishing defences against oil spills and net-capture. This is particularly 
true for those seabirds that have low fecundity and require a long period 
before attaining breeding age. For some species, continued survival may 
depend on legislated and enforced conservation and on protection of fish 
stocks and nesting and breeding habitats. 


Fish constitute the largest and most diverse group of marine vertebrates. 
They are taxonomically separated into the following three classes: 


Class Agnatha. This class encompasses the most primitive of living fish, 
the jawless lampreys and hagfish. This group evolved about 550 million 
years ago in the Cambrian, but presently has only about 50 species. 

Class Chondrichthyes. The sharks, skates, and rays which belong to this 
class are also known as elasmobranch fish; they are characterized by having 
a cartilaginous skeleton and lacking scales. This is also an ancient group, 
first appearing about 450 million years ago, and there are presently about 
300 species. 

Class Osteichthyes. This class ineludes the teleost fish which have a 
bony skeleton. This is the most successful group and comprises the vast 
majority of living fish, with somewhat over 20000 marine species. The 
teleosts evolved about 300 million years ago. 


The bodies of hagfish (Figure 6.4a) and lampreys are elongate and eel-like, 
and the animáis lack scales. The mouth is surrounded by a sucking disk, and 
most of the species are predators on other fish. The scavenging hagfish 
burrow into the bodies of dead or dying prey to feed on internal parts; 
lampreys are parasites which attach themselves to fish by their sucker-like 
disk and cut into the flesh to feed on soft parts and body fluids. All hagfish 
species are marine, whereas the different species of lampreys may be either 
marine or freshwater. Even the marine species of lampreys spend part of 
their lives in freshwater; the young live in rivers where they feed on small 
invertebrates and possibly fish fry, and after metamorphosing, they move to 
the sea to complete their development. 


Sharks (Figure 6.4c) are typically thought of as fast-swimming voracious 
predators that consume large prey, but many also act as scavengers in the 
sea. Paradoxically, the largest members of this group are docile feeders on 
plankton; these inelude the basking shark (Cetorhinus maximus) and whale 
shark (.Rhincodon typhus) which attain lengths of 14 m and 20 m, 
respectively. Both species have small teeth and strain plankton from water 
using specially modified gills. Skates and rays have flattened bodies, and 

Figure 6.4 Primitive fishes belonging to the 
Class Agnatha (a) and Class Chondrichthyes (b 
and c). (a) hagfish; (b) manta ray; (c) shark. 


most are adapted to a bottom-dwelling habitat. The majority are predators on 
benthic organisms (especially crustaceans, molluscs, and echinoderms), but 
some feed on fish, and the large manta rays (Figure 6.4b) are plankton 

Sharks and rays typically have internal fertilization and low fecundity, 
producing only small numbers of relatively large eggs. Most sharks and all 
rays give birth to Uve young. Skates lay their eggs in protective cases that 
are attached to a substrate, and the young hatch from these within a few 
weeks or months. 

There is an increasing demand for shark meat and shark fins, the latter being 
considered a delicacy in Asia. Numbers of sharks have sharply and rapidly 
declined in regions where fishing has intensified, and many sharks are also 
captured incidentally in commercial fishing operations for other species. 

QUESTION 6.4 Why are shark pop u latió os likely lo be slow to recove r from 


The many species of teleost fish inhabit di verse types of marine 
environments, and consequently they are a heterogeneous group in terms of 
anatomy, behaviour, and ecology. 

The most familiar teleosts are those that are harvested commercially 
(Figure 6.5a-f; Figure 6.9) and, because of their economic importance, more 
is known about the biology of these particular species. Such fish feed on 
many different prey Ítems depending on their size, location, and the 
availability of prey at different times. Some are strictly planktivores 
(plankton-feeders), other fish are piscivores (fish-eaters), or a combination of 
both. The most numerous of these fish occupy lower trophic levels; these 
inelude herring, pilchards or sardines, and anchovies, all of which eat chiefly 
zooplankton, although adult anchovies can feed directly on large 
chain-forming diatoms as well. Larger fish, such as cod, hake and pollock, 
may begin their lifes by feeding on small zooplankton; as juveniles, they 
switch to preying on larger zooplankton (e.g. euphausiids), and then become 
piscivores as adults. The largest of the pelagic teleosts are piscivorous 
species such as tunas, jackfish, and barracuda. Some fish, such as cod, 
haddock and hake, feed both in mid-water and on the sea bottom and are 

Figure 6.5 Commercially harvested types of 
teleost fish (a-f) and squid. (a) anchovy; 

(b) capelin; (c) cod; (d) pollock; (e) halibut; 
(f) tuna; (g) squid. 


capable of catching fish or benthic invertebrates. True demersal fish spend all 
their lives on or near the sea bottom, where some (e.g. solé) feed only on the 
benthos (clams, worms, and crustaceans being favoured foods) and others 
(e.g. halibut, turbot) eat smaller fish. 

Food supplies for oceanic fish vary in abundance due to physical factors. 
Some species respond to predictable seasonal variability in food 
concentration by migrating to certain feeding sites when prey becomes 
particularly abundant. For example, the migrations of tuna in the Pacific put 
them in areas where swarms of pelagic crabs are seasonally available. 
However, for many fish, variability in food concentration may cause 
significant change in their growth rate and survival, and this is reflected in 
the variability in fish catch from year to year. 

Fish associated with special benthic habitats, such as coral reefs, are 
themselves specialized to feed on coráis or resident plants and animáis; these 
species are considered in more detail in Section 8.6. As with most other 
animal groups, the largest fish populations are found in températe waters, but 
species diversity is much higher in tropical and subtropical waters. 

How do mesopelagic and balhypelagic teleosts di fíe r from epi pelagic 

Fish residing in deeper waters (>300 m) are not as numerous as epipelagic 
species, and they are not exploited commercially. The most diverse of the 
roughly 1000 mesopelagic fish species, both in numbers of species and 
individuáis, are the 300 + species of stomiatoids (Figure 6.6a-c, f, g) and 
the 200-250 species of lantern-fish (also called myctophids) (Figure 6.6d, 
e). Bioluminescence (see Section 4.4) is common in both groups, and the 
ñame Tantern-fish’ refers to light production from numerous photophores 
that are arranged in specific patterns in the different species of myctophids. 
Stomiatoids are distinguished by having photophores arranged in definite 
rows, as well as by having the dorsal fin located far back on the body. The 
photophores in both groups of fish contain symbiotic bacteria that produce 
the light which may be used to lure or lócate prey, or to find mates in these 
dark depths. 

The majority of mesopelagic fish are small, ranging from about 25-70 mm 
in length at maturity; the largest mesopelagic species are about 2 m long. 
Many of the stomiatoids have elongate, relatively streamlined bodies, but the 
hatchet-fish with their large, upwardly-directed eyes are named for their 
laterally-flattened and squared shapes (Figure 6.6c, g). Stomiatoid fish 
typically have large jaws with numerous sharp teeth, and they feed on 
zooplankton, squid, and other fish. Some species have the capacity to 
unhinge their jaws in order to ingest large prey (Figure 6.7), and many 
species have extensible digestive organs to accommodate large food Ítems 
(Figure 6.8). The best known stomiatoid genus, Cyclothone (Figure 6.6b), 
contains many species, and these fish live between 200 m and 2000 m depth 
in large schools. The shallower-living species are silvery or partly 
transparent; the deeper residents are typically black. The lantern-fish perform 
diel vertical migrations, some rising to the very surface to feed on planktonic 
crustaceans and chaetognaths, and this group comprises a major food source 
for tuna, squid, and porpoises. Lantern-fish range in size from 25 mm to 
250 mm in length. Many display sexual dimorphism in the arrangement of 
photophores, suggesting that they recognize sex by male-female differences 
in light patterns. 


Figure 6.6 Mesopelagic (a-g) and bathypelagic (h-j) fishes. (a) Vinciguerra attenuata, a 
stomiatoid. (b) Cyclothone microdon, a stomiatoid. (c) Argyropelecus gigas, a hatchet-fish. 
(d) Myctophum punctatum, a lantern-fish. (e) Lampanyctus elongatus, a lantern-fish. 

(f) Bathophifus longipinnis, a stomiatoid. (g) Argyropelecus affinis, a hatchet-fish. 

(h) Eurypharynxpelecanoides, a gulper-eel. (i) A female Ceratias holboelli with an attached 
parasitic male (j), deep-sea angler-fish. 


Figure 67 Swallowing mechanism of the 
stomiatoid Chaufiodus sloani, a deep-sea viper 
fish. (a) The positions of the skull and jaw 
bones when the mouth is open and closed. 

(b) The attitudes of the fish when ingesting 

In bathypelagic waters (below 1000 m), there are about six times fewer fish 
species. The greatest diversity is found in the 100 or so species of ceratioid 
angler-fish, so-named because of the characteristic bioluminescent lures 
which the females dangle in front of their mouths. Population sizes also 
diminish in deeper water and, as potential mates become more difficult to 
find, some fish (and also some invertebrates) exhibit reproductive and 
development patterns that differ considerably from those of shallower-living 
species. One extreme strategy has developed in some of the angler-fish in 
which young males live freely, but later attach themselves to females 

Figure 6.8 The ingestión capability of 
deep-sea fish. (a) Chiasmodon niger with a 
curled-up fish in its stomach that is longer than 
itself. (b) EvermanneUa atrata containing a 
squid. Sizes range up to 150 mm long. 


(Figure 6.6i, j). The males undergo a morphological transformador! and 
remain small (about 15 cm long); they Uve as an external parasite on the 
much larger ( ca . 1 m long), free-living female and serve only to fertilize her 
eggs. Gulper-eels ( Eurypharynx ) (Figure 6.6h) are also residents of the 
bathypelagic zone. These dark-coloured, elongate fish, with a funnel-like 
throat, attain lengths of 1-2 m and are capable of swallowing large fish 
as prey. 

In contrast to elasmobranchs, almost all teleosts have external fertilization 
and high fecundity (see Section 4.3). Whereas some species attach their eggs 
to a substrate, most lay large numbers of small floating eggs, and the 
hatching larvae form part of the meroplankton. Teleosts typically spawn 
many times, and growth is continuous through Ufe. These characteristics 
make them less vulnerable to commercial harvesting than the cartilaginous 
sharks and rays. 


The swimming abilities of most epipelagic fish make them independent of 
ocean currents, and they are able to migrate from one area to another, 
selecting favourable conditions in terms of food availability or reproductive 
sites and associated physical parameters. Whereas many species may 
undertake oceanic migrations ranging from several hundred to several 
thousand kilometres between, for example, feeding and spawning areas (see 
Figure 6.14), other fish may undertake migrations between the sea and 

Anadromous fish, such as salmón, sturgeon, shad, smelt and sea lampreys, 
breed in freshwater. The young then migrate to sea, where they spend most 
of their adult life. The length of time spent at sea is species-specific, but the 
adults eventually return to their specific freshwater sites to breed and spawn. 
Some species, like the Pacific salmón, die after mating; but others, such as 
the Atlantic salmón, do not and may return several times to their 
breeding site. 

Catadromous fish are those that breed in the sea, but spend the majority of 
their adult life in freshwater. Some of the longest migrations are undertaken 
by the catadromous American {Anguilla rostrata) and European ( A . anguilla) 
eels. Adults migrate from rivers in Europe and eastern North America to 
breeding sites in the Sargasso Sea, where they spawn many small floating 
eggs in deep water and then die. The larvae remain at sea for one or two 
years before arriving at the coasts of America and Europe, respectively; 
there they metamorphose into elvers which enter the estuaries and freshwater 
rivers. They remain for 8-12 years in their freshwater habitats before 
returning to the sea as mature adults. 


Marine fisheries constitute a multibillion-dollar industry supplying about 20% 
of the animal protein consumed by humans, and also producing animal feeds 
for domestic livestock and poultry, fish oils for paints and drugs, pet foods, 
and some food additives. As the human population continúes to expand, the 
increasing demand for high-quality protein and other marine resources has 
focused attention on the present stocks of commercial marine species and on 


the feasibility of increasing, or at least maintaining, the present harvest. It 
has become apparent that fisheries management has not always been 
successful in maintaining fish yields and conserving stocks (see Section 9.1), 
and that our information concerning the biology and ecology of many 
species may be insufficient to establish reliable estimates of yields. One 
branch of marine Science, fisheries oceanography, addresses these problems. 


In an evolutionary sense, the most successful of the larger marine animáis 
are the extremely abundant species of fish that are hunted commercially. 
These inelude herring, anchovies, sardines, cod, and mackerel, some of 
which are illustrated in Figures 6.4 and 6.9. These fish, and others, are 
among the top ten species of fish that make up the world fish catch 
(Table 6.2); Table 6.3 lists the major fishing nations of the world. The total 
world reported catch of marine fish (including shellfish and squid) in 1993 
was 84 million tonnes, down from a high of 86 million tonnes in 1989. At 
least a further 27 million tonnes of by-catch - unwanted marine species 
caught incidentally - is thrown back into the sea every year, most of it 
dead or dying. The largest fraction (64%) of the global marine catch comes 
from the Pacific Ocean, with 28% from the Atlantic and 8% from the 
Indian Ocean. Humans directly consume about 70% of the total world 
catch (in live weight units); the remaining 30% is used as poultry or 
livestock feed, and this comes mostly from the smaller species of fish such 
as anchovies, herring, and sardines (or pilchards). 

Table 6.2 Principal species of fish comprising the total world fish catch (FAO 
statistics, 1993). Relative dominance of species may change from year to year due to 

climate change and/or exploitation pressure. 

Species 1993 catch 

(x 10 6 tonnes) 

Peruvian anchoveta ( Engraulis ringens ) 8.3 

Alaska pollock ( Theragra chalcogramma ) 4.6 

Chilean jack mackerel ( Trachurus murphyi) 3.4 

*Silver carp ( Hypophthalmichthys molitrix) 1.9 

Japanese pilchard ( Sardinops melanostictus) 1.8 

Capelin (Mallotus villosus) 1.7 

South American pilchard ( Sardinops sagax ) 1.6 

Atlantic herring ( Clupea harengus) 1.6 

Skipjack tuna (Katsuwonus pelamis) 1.5 

* Grass carp ( Ctenopharyngodon idellá) 1.5 

* Freshwater and cultured species 

QUESTI0N63 In Table 6.3 how can yon explain the high fish catch of two of 
the world's smaller countries. Chile and Perú? ( Refer to Section 3,5 if 

The management of the world’s fishing industry is very complicated because 
it involves not only biological and econological knowledge of many species, 
but it also must take into account economic considerations, competition 
between nations, labour unions, and public marketing strategies. It is beyond 
the scope of this text to deal with economic and political problems in 
fisheries, but it is germane to consider oceanographic topics that may supply 
explanations for comparative abundance of fish species and for fluctuations 
in fish populations (as shown, for example, in Figure 6.9 and 6.10). 


Table 6.3 Principal fishing nations of the world and 1993 FAO catch statistics 
(including aquaculture). Relative dominance of nations may change from year to 
year due to ecological or economic changes. 


1993 catch 
(xlO 6 tonnes) 











Russian Federation 








Korean Republic 






*Approximately 50% of the fish catch from China is from aquaculture 

The history of fisheries Science has been briefly reviewed in Section 1.4. Early 
studies on the population dynamics of fish stocks led to the development of 
what are generally referred to as stock/recruitment theories, ‘stock’ referring 
to population numbers of adult fish, and ‘recruitment’ to the numbers of 
juvenile fish entering the adult population. These fisheries theories were based 
upon a central premise that reproduction, survival, and productivity of fish 
populations were largely independent of changes in the physical environment 
of the fish, or of changes in biological components (i.e. interacting species) 
within the community under consideration. The basic argument put forward 
was that the recruitment of new fish stock was a function of the numbers of 
eggs produced and subsequent survival of young. Because total egg 
production is a function of the size of the adult population and survival was 
considered constant, it was maintained that the size of the adult stock could 
be controlled by manipulating fishing pressure through regulating the 
number of boats, the size of nets, and the total allowable catch. This basic 
premise, with later variations, became the basis for the management of 
fisheries for nearly 100 years. 

In cases when reliable assessments can be made of the numbers of juvenile 
fish that will enter a fishery within the next one or two years, the stock/ 
recruitment approach has been of some valué. On the whole, however, 
fisheries scientists have not been successful in managing fish stocks, or in 
making long-term predictions. It is now clear that the abundance of fish is 
determined by a variety of factors, and data are being compiled from a 
number of fields to answer fundamental questions about fish ecology. 
Plankton ecology may provide some answers, as most fish eggs and fish 
larvae are meroplanktonic, and many adult fish depend on the plankton 
community for food. Changes in the physical environment may also have 
strong influences on fish populations. Fisheries oceanography concerns the 
search for knowledge about the natural regulation of fish populations and 
seeks to apply this information to fisheries management. 


Changes in the abundance of some species of fish are associated with long- 
term changes in the oceanic climate (see Section 4.8). Figure 6.9 shows a 



Figure 6.9 The Russell cycle in the western 
English Channel: Long-term fluctuations in the 
abundance oí herring ( Clupea harengus), 
pilchards (Sardina piichardus), and mackerel 
(Scomber scombrus) as related to changes in 
oceanographic climate and the ratio oí 
numbers oí the chaetognaths Sagitta elegans and 
Sagitta setosa. (Arbitrary units) 

regime shift, known generally as the Russell cycle, that has taken place in 
the western English Channel. Herring decreased in abundance from about 
1930 as water temperatures warmed. At the same time, pilchards started to 
become more abundant, reaching their highest populations during the 1940s 
and 1950s when herring were scarce. As water temperatures cooled in the 
1960s, pilchards disappeared from the area, and mackerel became the 
dominant pelagic fish. During the same periods, there was a change in the 
dominant chaetognath species, from Sagitta elegans during cool intervals to 
S. setosa in warmer water. It is generally agreed that this is a natural cycle 
caused by climate change, and that the changes in fish species are 
independent of fishing activity. 

Figure 6.10 presents the history of the Peruvian anchovy catch as an exampíe 
of a managed fishery that failed to take natural environmental change into 
account. Waters off Perú are normally enriched by Coastal upwelling that 
leads to very high productivity (see Section 3.5.2) and a short food chain 
(Figure 5.3). Until about 1970, enormous numbers of plankton-eating 
anchovies were produced, and these were the major food for millions of 
seabirds (boobies, brown pelicans, gannets, and cormorants being dominant). 
The birds are collectively referred to as guano birds, the Spanish ñame 
referring to the faecal droppings of the birds. Guano from the birds built up 
over time on their nesting islands to depths of up to 50 m and, because it was 
rich in nitrates and phosphates, it was collected and sold for fertilizer. 

The Peruvian anchovy fishery was developed in the late 1950s and was to be 
a model of fisheries management. It was predicted, based on stock size, that 
between 9 and 10 million tonnes of anchovies could be harvested annually 
without decreasing the stock. Figure 6.10 shows the increase in annual fish 
catch from less than one million tonnes in 1958 to 13 million tonnes in 


Figure 6.10 Changes in the guano bird population and the Peruvian anchovy (Engraulis 
ringens) catch along the west coast of South America. Arrows indícate El Niño years and the 
relative intensity of these climatic changes. Anchoveta numbers represent the total catch 
from Perú, Chile, and Ecuador. 

1970, making it the largest fishery ever based on a single fish species. 
However, the fishery collapsed in the 1970s and it has taken 20 years to 
recover to an annual catch of about 8 million tonnes of fish (see Table 6.2). 
One of the factors that was not fully understood in the 1960s and 1970s 
when this fishery was being ‘managed’ was the effect of El Niño events on 
the anchovy stock. 

El Niño is the ñame given to a warm, nutrient-poor surface current that 
flows over the coid Coastal upwelling off the coast of Perú. The phenomenon 
has been well documented for about 50 years, and was recorded for 
hundreds of years before that. El Niño events occurred in 1957-58, 1965, 
and 1972-73 (Figure 6.10). Each time they occurred, there was a decrease in 
the guano bird populations which fed on the anchovy. With the intrusión of 
warm oligotrophic surface waters, the anchovy migrated deeper in the 
water — too deep to be reached by these diving birds, and many birds 
starved. In 1957, up to 20 million birds died during the El Niño, but 
recovery of bird populations following natural El Niño events was usually 
relatively rapid. However, the fishery continued during these El Niño 
episodes in spite of the fact that the anchovy were being displaced from their 
natural, near-surface habitat. A lack of understanding the effects of El Niño 
on the anchovy, coupled with overestimates of máximum sustainable yield 
as calculated from stock/recruitment theory, led to the eventual depletion of 
the stock. The fishery harvested less than 2 million tonnes of anchovy from 
1977 to 1985. Bird populations dropped to about one-tenth of their original 
numbers and, although fish are now increasing in number, the seabirds have 
not recovered from levels of about 3 million. 

The examples of the Russell cycle and the Peruvian anchovy fishery have 
been given to demónstrate that environmental changes may significantly 
affect both the yield of fish and the type of fish present in any area. When 
overfishing is coupled with a natural environmental change that is also 
decreasing the stock, the consequences may be severe and the recovery of 

anchovv catch (x 10 6 ton ñas) 


the stock may take decades, if it occurs at all. It is now clear that the role of 
the environment cannot be excluded from fisheries theories, and that 
oceanographic data may contribute useful and necessary information to 
create new management theories. Because marine fish stocks occupy large 
expanses of ocean, satellite remóte sensing is now being applied to examine 
those variations in ocean conditions that cause natural fluctuations in the 
distribution, abundance, and availability of commercial stocks. 


Most teleost fish species are highly fecund, each female usually spawning 
between 10 3 and 10 6 eggs per year. An extremely small variation in the 
mortality of the progeny (e.g. from 99.90 to 99.95%, see Question 6.11) may 
cause a very large change of several hundred per cent in the adult population 
size. Thus it is important that fisheries management theories inelude an 
ecological understanding of the factors controlling the survival and 
recruitment of larval and juvenile fish into adult stocks. Factors determining 
growth rates of fish are also important because they determine size and 
partly control survival in young fish. 

A number of hypotheses have been formulated to explain fluctuations in 
adult abundance due to differences in the recruitment and growth of young 
fish, and none are mutually exclusive. Some of these hypotheses are listed as 
folio ws: 

1. Starvation hypothesis. If there is not enough planktonic food in the 
sea, larval fish mortality will increase and few, if any, will survive to 
become adults. 

2. Predation hypothesis. Predators, including larger fish and some 
carnivorous zooplankton, may consume large numbers of both larval and 
juvenile fish. Heavy predation results in few young surviving to become 

3. Advection hypothesis. Physical oceanographic processes may transport 
the young fish away from their nursery areas to unfavourable environments 
where they will not survive. 

4. Growth hypothesis. The máximum size attained by fish at the time of 
harvest, multiplied by the number of fish captured, gives the biomass yield 
to the fishery. Size and numbers of fish are also used to establish fish quotas 
in terms of allowable tonnage. Numbers are determined by survival (see 
hypotheses 1-3 above), and size is determined by growth. The growth 
hypothesis is based on the consequences of fish growth being inhibited by 
either biotic (e.g. food) or abiotic (e.g. temperature) factors. 

In order to see how these hypotheses might be integrated with present 
biological oceanographic knowledge, several mechanisms of fish recruitment 
and growth are discussed below. 

The tendeney of teleost fish to produce very large numbers of eggs is quite 
unlike the reproductive patterns of most terrestrial animáis of similar size. A 
single female cod, for example, may lay more than a million eggs per year. 
Obviously only a very small percentage of these eggs are needed to replace 
the adult stock. The mortality of eggs and fish larvae is very high, as 
illustrated in Figure 6.11, and may result from predation, advection, or, in 
the case of larvae only, from starvation. 


Figure 6.11 Idealized population mortality and 
individual growth curves for a species of teleost 
fish during its lite cycie from egg to adult. 
Arbitran/ units of age and length, and variable 
time intervals (indicated by dashed lines) for life 

1 The hypothesis that starvation may regúlate larval fish survival is 
concerned with what is calleó the critical phase in the life of fish. This 
phase begins immediately after hatching when the young larvae still have the 
remnants of the yolk sac on which to subsist. In order to survive, the larvae 
must begin to eat sufficient planktonic food before the yolk is exhausted. 
This means that the larvae must hatch at a time in phase with abundant 
plankton concentrations. If a larval fish hatches too early, or too late, relative 
to its food supply, it will die (Figure 6.12). 

2 The predation hypothesis assumes that larger organisms have fewer 
predators and are also better able to escape from predatory attacks, Thus if 
larval or juvenile fish can grow fast enough, mortality will be lessened and 
more fish will survive. This is illustrated in Figure 6.13 which also shows 
that this hypothesis is partly dependent on the planktonic food supply, 
although here food concentrations are not restricted to the critical phase as in 
Figure 6.12. 

QUESTION 6.6 (a) Referring to Figure 6.13b, is ihere a growth rale for young 
fish below which there would theoretically be 100% mortality due to 
predation? (b) Would you ex pee t to find 100% mortality from predation in 
nal u re? 

3 The advection hypothesis can be illustrated in a number of different ways 
depending on the type of fish species. For example, plaice tend to spawn on 
specific sites that are associated with favourable nursery areas to which the 
larvae are carneó by cunents. This is shown in Figure 6.14 for a population 
of plaice that spawns in the Southern North Sea. In some years, however, 
strong storm activity may disrupt the current System that canies larval fish to 
their nursery area, and the plaice larvae may then be transponed to areas that 
are unfavourable for survival. 

4 The growth hypothesis derives from the growth curve shown in 
Figure 6.11. Growth is dependent on a variety of parameters affecting the 

Figure 6.12 The critical phase of larval fish survival requires that planktonic food (often 
copepod nauplii) must be present in the water at the time of hatching (e.g. at time A), it the 
food organisms occur later (e.g. at time B), all the fish larvae from one particular spawnmg 
will die from starvation. (Arbitrary units.) 



(a) (b) 

Figure 6.13 (a) A hypothetical relationship showing the difference in growth rates (2% to 
8% per day) of juvenile fish depending on the concentraron of planktonic food (usually, 

(b) The growth rates from (a) are shown over time, together with the size at which maximal 
predation of the fish takes place. Note that this size also increases slightly with time as the 
predators themselves grow. Juvenile fish that grow slowly (e.g. at 2% day" 1 ) are exposed to 
predation over a longer time period and therefore have lower survival rates than 
faster-growing (e.g. 8% day -1 ) fish. (All units are arbitrary valúes.) 

Figure 6.14 The larval drift and migrations oí 
plaice (Pieuronectes platessa) in the North Sea. 
Larvae hatch in the spawning area, then drift 
northeast to a food-rich nursery area. They 
remam in the nursery area for their first year of 
Ufe, growing from 1.5 cm to 20 cm in length 
before migrating to the northwest. 

rate of growth and the length (size) at maturity. In general, the growth rate is 
directly proportional to temperature, but size at maturity is inversely 
proportional to temperature. Thus an increase in temperature, which is 
governed by the physical ocean climate, can have the dual effect of 
producing more rapidly growing fish, but ones that are smaller at maturity. 

In addition, the growth efficiency (K\ or K 2 , see equations 5.10 and 5.11) of 
fish varíes with the type of food consumed. Prey with high protein content 
(e.g. copepods) produce faster growth than foods (e.g. small ctenophores) 
with very high water content and low protein. Growth is also influenced by 
the metaboiic costs associated with particular types of prey; for example, a 
predator that has to chase its prey would have a higher metaboiic cost than 
one that filters its food. Thus, as prey type changes (due to changes in the 
ecosystem), the growth efficiency of fish will also change, and this will 
affect the growth curve (Figure 6.11). 

In summary, each of the four hypotheses discussed above can be shown to 
have some experimental support. However, none can be shown at present to 
be the only mechanism determining the fluctuations in the abundance of fish, 
and it is likely that more than one mechanism is operable. It is also possible 
that some other factors, such as fish disease, may at times be important in 
regulating recruitment of young to adult stocks. In order to improve the 
management of fisheries, these mechanisms need to be further researched 
through experimentation, field data, and ecosystem Computer models. 

QUESTIQN 6.7 Does Figure 6.11 suggest that. in order to increase the biomass 
yield of fish, it is better for a fishery lo al low larger numbers of small fish to 
survive, or smaller numbers of large fish? What other factors might affect 
the decisión? 



The previous section has concentrated on mechanisms that might be relevant 
in producing seasonal and long-term variations in the abundance of fish. 
Another problem faced by fishermen is where to find the main concentrations 
of fish. In general, fishermen have been almost too successful in finding fish, 
and some stocks have been seriously depleted due to the massive investment 
in the mechanized harvest of fish. However, it still remains a problem to 
forecast the exact location of fish schools on time scales that relate to the 
time which a fishing boat can economically spend at sea. 

Certain types of oceanographic data, such as surface temperatures and depth 
of the thermocline, can greatly assist fishermen in rapidly locating fish and 
in reducing the cost of remaining at sea, but only if these data are collected 
during a time that is near to the harvest period. This type of information is 
referred to as ‘near real-time data’; the data are collected very cióse to the 
time of an event for which the data are required. 

Fish become easy targets for fishermen when they are concentrated into 
schools. They may form schools when feeding, or during periods of 
reproduction and migration. Feeding schools are often associated with very 
productive waters, such as might be present on a particular bank (i.e. a 
shelf-break frontal zone as defined in Section 3.5.4), or at a boundary 
between two water masses (e.g. a planetary frontal zone, Section 3.5.3). 
Reproduction may take place in other localities, with the fish migrating 
together between feeding and spawning sites. 

The location of particular types of fish is often an important factor in the 
regulation of fisheries. For example, in the North Pacific Ocean, salmón 
generally stay in subarctic waters of <14°C, whereas the squid and tuna 
fisheries are in subtropical waters of >14°C. The boundary between these 
two water masses is not a fixed line, but varies geographically by several 
hundred kilometres depending on the physical oceanography. Near real-time 
oceanographic information on the location of this frontal zone is important 
to the high seas fisheries. This is particularly true for the squid fishery which 
is carried out in the open ocean and, by international agreement, is not 
allowed to stray into areas where salmón may be inadvertently collected as 
well. Ocean surface temperatures can be detected by satellites, and the 
location of the 14° boundary is relayed to the Japanese squid fishing vessels 
via a land-based station which also monitors the location of the fleet. 

It is also important for inshore fixed-depth fisheries to be able to predict the 
location of fish schools from near real-time oceanographic data. Figure 6.15 
illustrates the location of cod stocks relative to a trap fishery that is 
maintained at a fixed depth throughout the year. (This illustration serves as a 
good example because the location of the fishing effort is fixed every year; 
however, it could also apply to a net fishery operating at a fixed depth.) A 
trap, consisting of a large net that directs swimming fish into a central area 
from which they cannot escape, is set from the surface to the seafloor. 
Various physical disruptions in the water temperature can result in the 
location of the cod at very different depths and regions relative to the trap 
locations. Under some circumstances (conditions b and d in Figure 6.15), the 
position of the traps is such that few, if any, fish would be captured. At such 
times, fishermen might profitably employ a different type of fishing method, 
using nets towed from boats or, in the case of condition ‘d\ the traps might 
be relocated in deeper water if feasible. The application of near real-time 


Figure 6.15 The trap fishery ¡llustrated here (and other inshore fisheries) can be severely 
affected by local water temperature conditions, which in turn are determined by prevailing 
wind and air temperature conditions. (a) shows a June situation in an area having a late, 
coid spring with light variable prevailing winds. The surface water has warmed enough to a 
depth of about 18 m to allow cod to come cióse inshore at shallow trap depths. In 
(b), prevailing offshore winds have driven the suitable water layer off the shore, and coid 
water coming to the surface severely restricts the area in which trap fishing might be 
successful. In (c), prevailing onshore winds early in the season have had the opposite effect, 
pushing the suitable water layer onto shore, and deepening and expanding the potential for 
good trap fishing. Finally, in (d), a late-season situation is shown in which prolonged warm 
weather and onshore winds have combined to produce a nearshore layer of water too warm 
for cod; the fish stay below it out of reach of the trap. 

data on the thermal structure and movement of the water masses near the 
traps could greatly assist this fishery, and such information should become 
an integral part of fisheries management. 

As mentioned earlier, fish schools also form during reproductive periods, and 
the congregation of herring on certain banks in the North Sea is a 
well-known phenomenon associated with the spawning of these fish. 
Unfortunately, it has also become the time and place where the herring are 
most easily caught — just when they are about to create a new population. 
There is obviously a división of opinión in such a case as to whether one 
catches the fish when it is most economical to do so, or whether it would be 
better to wait for a time when less biological damage would be done. 

The migration routes of fish can also be greatly influenced by local 
oceanographic conditions, which in turn affect the fishery. An example from 
North America is given in Figure 6.16, illustrating migration routes of Fraser 
River sockeye salmón off the west coast of Cañada. During 1955-77, most 
of the fish returned to the river through the Strait of Juan de Fuca, where 
they were exposed to both Canadian and U.S. fishermen. However, during 
1978-83, up to 80% of the fish returned through routes north of Vancouver 
Island, where they were available only to Canadian fishermen. This resulted 
in a change of considerable economic impact to the fisheries of the two 
adjoining nations. A Computer model that used the temperature and salinity 
of Coastal water (the latter indicating amount of freshwater inflow from 
rivers) was developed to predict the diversión in 1978-83 as indicated in the 
inset graph. 


Figure 6.16 The migration routes of sockeye 
salmón returning to the Fraser River (British 
Columbia, Cañada) for spawning. The inset 
graph shows the percentage of fish that were 
diverted through Johnstone Strait during return 
to the Fraser River from 1955 to 1988 (solid 
line). From 1955 to 1977, most fish returned to 
the river through the Strait of Juan de Fuca; 
from 1978 to 1983, the majority of the salmón 
returned through Johnstone Strait. The dashed 
line in the inset graph indicates predicted 
diversions through Johnstone Strait based on a 
Computer model simulating changes in 
freshwater flow and water temperature. 


One way of managing fishery resources and increasing yield is to attempt to 
control many environmental variables by growing marine species in 
enclosures or impoundments. Such systems, because they concéntrate the 
target species and are located in Coastal areas, also have the advantage of 
easy and economical harvesting. The cultivation of marine species is called 
mariculture. It can be regarded as the marine counterpart of agriculture, as 
many of the principies and problems associated with increased production 
through artificial culture are similar. Interest in mariculture continúes to 
increase as the human population expands and demands more protein, and as 
open-sea fishing reaches the limits of exploitation of wild stocks. 

A variety of pelagic and benthic organisms are cultured including some 
seaweeds, crustaceans (shrimp or prawns), molluscs (mussels, oysters, 
scallops, clams, and abalone), and fish (e.g. salmón, mullet, solé, turbot, 
eels). Most of these species are raised for human consumption, but some are 
cultured for other commodities, such as pearls, food additives (e.g. alginates 
from seaweeds), and domestic animal feed. 

One of the simplest forms of mariculture involves the transplantation of wild 
stocks (e.g. fish, oysters) into new areas where natural conditions are 
favourable for increased production. Successful transplants have involved the 
introduction of a Pacific salmón into Lake Michigan in the U.S. and into 
New Zealand and Chile. Another very simple technique is the trapping of a 
wild population in enclosures, where the animáis are held without artificial 
food until harvesting; this is done with shrimp in Singapore and some fish 
(e.g. mullet) at various localities. More intensive culture requires more 
manipulation in terms of providing feed and fertilizers, controlling the 
physical environment, and eliminating predators and disease. 


The most economical species to culture are those that occupy lower trophic 
levels, feed on naturally available food, and can produce high biomass in 
crowded conditions. Mussels, for example, feed by filtering plankton, and 
they are routinely cultured in many Coastal areas. In nature, mussels attach to 
hard substrates and their numbers are often controlled by the amount of 
space available and by the degree of predation caused by starfish and boring 
snails. In culture, mussels are provided with ropes suspended from rafts for 
attachment, thus increasing the space available and also eliminating benthic 
predators; the mussels feed on plankton in the surrounding water. Such 
intensive culture can yield up to 600 tonnes of mussels per hectare annually; 
about 50% of this is drained body weight, available for consumption. 

Animáis from higher trophic levels require the provisión of either artificial 
feed or of cultivated or captured prey and, because they are generally larger 
in size and mobile, they also occupy more space. However, the relatively 
high costs of culture may be justified by higher market valué. For example, 
salmón can be grown in enclosures (Figure 6.17) and fed on synthetic feed 
(which has the advantage of chemically known constituents) or on processed 
euphausiids captured at sea (see Section 6.1); the high expenses of this 
culture are recovered in the high market price of the salmón. 

Another form of salmón enhancement is to rear salmón eggs in hatcheries 
and then release the young salmón into the sea. This is known as ocean 
ranching, from analogy with raising some cattle by releasing them into 
ranchlands. Depending on the species, released salmón spend between 2 and 
5 years in the open ocean before returning to the same stream in which the 
hatchery is located. Ocean ranching is less expensive than pen culture, but 
the retums of adult fish depend upon survival at sea. 

The examples given above all utilize only one species; this type of System is 
known as monoculture, and it represents an extreme simplification of a 
natural community. In order to increase profitability, some Systems attempt 
to culture more than one species in the same enclosure; this is known as 
polyculture. In a monoculture system, uneaten food and faecal material fall 
to the bottom where they are either flushed by natural water movement, or 
are otherwise cleaned from tanks to prevent excessive bacterial growth and 

Figure 6.17 An aerial view of a salmón farm 
in British Columbia, Cañada. The salmón pens 
are located in a protected cove that shelters the 
enclosures from storm damage. 


the formation of anoxic water. A polyculture system introduces species that 
feed on this detritus and can also be harvested for commercial profit. 
Herbivorous fish (e.g. some carp, mullet) are commonly grown in ponds 
together with prawns that feed on detritus, fishmeal, and/or filamentous 
algae. It is also possible to combine fish that feed on zooplankton with those 
that eat benthic plants. Polyculture is presently more highly advanced in 
freshwater systems, but it can be expected to expand as more mariculture 
systems are developed. 

The leading countries in mariculture tend to be those with high human 
populations and high protein demands. Fish (carp) culture began about 4000 
years ago in China, and this country continúes to lead the world in terms of 
quantity of production from combined marine and freshwater culture (see 
Table 6.3). The second leading country is Japan, which has developed some 
of the most advanced mariculture facilities and techniques. In North 
America, only about 2% of fishery producís come from culture, and that 
figure ineludes cultivated freshwater species as well as marine organisms. In 
Europe, Spain is the leading producer with mussel culture. On a global basis, 
roughly 5 x 10 6 tonnes of marine species are grown in culture annually. 

PUESTION 6.8 What is the perceniage of marine species produeed in culture 
compared with the total marine fish catch harvested from the oceuns? 

There is no doubt that mariculture will continué to expand throughout the 
world in response to increasing demands for more protein for human 
consumption and, to a lesser extent, to supply luxury foods such as salmón 
and lobster, or other commodities. There are several constraints to this 
expansión, however. Some problems are technological, like the selection of 
suitable sites or disease control, and some of the constraints are economic, 
balancing high costs of feed, fertilizer, and manpower against market prices. 
A much more serious problem looms with increasing Coastal pollution 
throughout the world. Mariculture depends on Coastal sites, whether cultured 
species are grown in enclosures in the natural environment or in land-based 
facilities that depend on water pumped from a nearby marine source. Eggs 
and young stages of marine animáis are particularly sensitive to pollutants, 
and adults may accumulate Chemical or biological substances from polluted 
water that make them dangerous to humans. Shellfish, for example, filter the 
cholera bacterium from sewage-polluted water, and consumption of infected 
animáis spreads and compounds this disease. 


1 The nekton comprises the larger, pelagic, marine animáis whose 
swimming abilities are such that their movements are independent of ocean 
currents. Included in this category are larger crustaceans (some euphausiids, 
shrimp, and swimming crabs), squid, sea snakes, marine turtles, and marine 
mammals, with adult fish making up the dominant fraction. Seabirds are also 
considered here because they are dependent on the sea for food and may 
have considerable influence on the neuston and epiplanktonic communities. 

2 Few commercially harvested crustaceans are pelagic, but some of the 
larger, very abundant euphausiid species are presently fished in the Antarctic 
and off Japan. The superabundance of the Antarctic krill (.Euphausia 


superba ) makes it an attractive fishery target, and it is likely that this harvest 
will increase, despite high economic costs. 

3 Squid form another abundant invertebrate group targeted by fisheries. 
More needs to be known about the biology and abundance of these animáis 
before fisheries management can be effective in protecting these stocks. It 
has become evident that driftnet fishing for squid is very unselective, and 
that vast numbers of seabirds, fish, turtles, and marine mammals have been 
inadvertently captured and killed by this method. More selective, alternative 
squid-fishing techniques are available, although their use increases the costs 
of fishing. 

4 Eight species of turtles, one lizard, and about 60 species of snakes are 
the only reptiles to have become marine. The turtles have become 
endangered species from hunting of the adults and their eggs. 

5 There are about 110 species of marine mammals. The largest are the 
baleen whales that feed by filtering zooplankton or fish (or benthic 
invertebrates, in the case of the grey whale) through their plates of baleen. 
The toothed whales (including dolphins and porpoises) are predators in the 
sea. These two groups of cetaceans together consume a much greater 
quantity of marine biomass than is removed from the oceans by the entire 
commercial fishery. 

6 Many species of whales, pinnipeds (seáis, sea lions and walruses), and 
sirenians (manatees and dugongs) have been extensively hunted. Their low 
fecundity and long development times from birth to maturity make them 
especially vulnerable to rapid depletion of population numbers through 
commercial harvests. Many of the species are now endangered, and recovery 
of populations is slow. 

7 The most highly adapted of the seabirds spend 50-90% of their lives at 
sea, but all remain dependent upon land for nesting sites. The highest 
numbers are found in association with very productive waters, where 
zooplankton and fish are concentrated. Non-migrating species are subject to 
natural mortalities caused by climate change and subsequent declines in their 
prey (e.g. the effects of El Niño on the guano birds off Perú). 
Human-induced mortalities such as overfishing, habitat destruction, 
introduction of predators, and Coastal pollution increasingly threaten seabirds. 

8 The great majority of marine fishes are teleosts with a bony skeleton, and 
the 20000 or so species show considerable diversity in terms of anatomy, 
behaviour, and ecology. The most abundant species are epipelagic 
plankton-feeders with very high fecundity, and many of them (e.g. herring, 
sardines, anchovies) form the basis for some of the most profitable marine 

9 There are fewer deep-water species of fish, and they are not as numerous 
as their shallower-living relatives. Many of the meso- and bathypelagic fishes 
tend to be relatively small. Many of the species have photophores and use 
bioluminescence to lócate or lure either prey or mates, or to evade predators. 

10 In 1993, the total world catch of all marine species of fish (including 
squid and shellfish) was about 84 million tonnes per year. About 64% of this 
catch was taken from the largest of the oceans, the Pacific Ocean, with 
China, Perú, and Japan being the leading fishing nations. 


11 Although various attempts have been made in the last 100 years to 
manage fisheries, these have met with little success. Many fish stocks or 
fishing regions have been depleted or are in danger of being overfished. 

12 Fisheries management has traditionally been based largely on 
stock/recruitment theories that have ignored the role of the environment in 
causing natural fluctuations in the numbers of fish. It is becoming 
increasingly apparent that the recruitment of larval fish into adult stocks can 
vary greatly depending on whether there is sufficient food for young fish, 
whether predation is high or low, whether larval fish are transponed by 
currents into unfavourable habitats, whether disease affects the population, 
and whether growth is slowed or hastened by temperature, food availability, 
or other factors. 

13 Biological oceanography can assist fisheries in two ways. The first way 
is by increasing our understanding of what factors cause natural fluctuations 
in the abundance of fish. The second way is in providing near real-time 
oceanographic data in connection with the actual process of fishing; better 
information on the location of fish schools can reduce the cost of the 

14 Mariculture is another way of increasing the yield of fisheries resources. 
At present, only a few of the many marine species are under culture, but 
cultivation is expected to expand from its present production of about 

5 x 10 6 tonnes per year. 

Now try the following questions to consolídate your understanding ofthis 

QUESTION 6.9 From a knowledge of primary productivity in re I ai ion to the 
physical movement of water (Section 3.5), what other locatíons in the oeeans 
in addilion lo those discussed in ihis Chapter might support good fishing? 

QUESTiOM 6.10 Can the total tonnage of ‘fish’ be increased by caiching 
smaller and smaller fish. or even by harvesting zooplankton? 

0UEST10N 6.11 Suppose eaeh fem ale fish in a particular species lays 10' eggs 
each year, In one year, 99.90% of the eggs or progeny die; in another year, 
the mortaiiiy is 99,95%. Assuming that in each case the remainder beca me 
mature adults, what would be the difference in the numbers of the adult 
populations resultíng from these two year classes? 

OüESTION 6.12 What might be some ecológica! consequenees of fisheries 
cominuing lo remove increasingly large numbers of fish and squid from the 

Q0ESTI0N 6.13 Other than selecting species for culture on the basís of trophic 
level posilion, what additional physiologtcal and biological lealares might 
make certain organisms more amen able and attractive as possi hie culture 
organisms than others? Considcr the requirements of al! lile stages in 
fonrnjlating your answer. 

QUEST10N 6.14 What percentage of the world's ocean is now a designated 
whale sanctuary? íCónsul! Table 5A.) 



Relative to the pelagic zone, the seafloor presents a greater variety of 
physically diverse habitats that differ from each other in terms of depth, 
temperature, light availability, degree of immersion (tidal vs. subtidal), and 
type of substrate. Hard, rocky substrates provide sites of attachment for 
sessile species like barnacles and mussels which remain in one place 
throughout their adult life, and they provide crevices and depressions that 
can be used by mobile animáis as refuges from predators. Soft-bottom 
substrates (e.g. mud, clay, sand) offer both food and protection for 
burrowing animáis. At least partly owing to the greater variety of benthic 
habitats, the number of species of benthic animáis (estimated at > 1 million) 
is much greater than the combined number of pelagic species of larger 
zooplankton (about 5000), fish (>20000) and marine mammals (ca. 110). 

As in the pelagic environment, vertical gradients of temperature, light, and 
salinity are especially important in establishing distinctly different living 
regimes for benthic organisms. Figure 1.1 shows the ecological divisions of 
the seafloor based on depth and topography. Some of the ecological-depth 
divisions have well-defined boundaries, others are more arbitrary zones, but 
each of these benthic habitats presents distinctly different living conditions. 
The animáis that inhabit different zones will generally be of different species, 
each uniquely adapted to the particular environment in which it is found. 

The smallest benthic zone (Figure 1.1) is the supralittoral or supratidal 
zone, an area just above high water mark and immersed only during storms. 
On steep shores, this zone will receive spray from breaking waves, and it is 
sometimes referred to as the ‘splash’ zone. On fíat beaches, the area may be 
marked by heaps of seaweeds cast ashore. Few species are adapted to live in 
this transitional región between the sea and land. 

The littoral or intertidal zone lies between tide marks and is thus immersed 
at high tides and exposed at low tides. The extent of this zone depends upon 
local topography and tidal range. This area lies within the euphotic zone, and 
benthic algae as well as phytoplankton are available for grazing and 
filter-feeding benthic herbivores. These in turn support a diverse and 
abundant carnivore community. 

The sublittoral (subtidal) zone extends from the low tide mark to the outer 
edge of the continental shelf, at a depth of about 200 m. Part of the 
sublittoral area also lies within the euphotic zone, but benthic plants decline 
from low numbers to zero in the deeper regions. Rocky substrates become 
scarce and are replaced by soft substrates. The sublittoral zone occupies 
about 8% of the submerged seafloor. 

The remaining benthic habitats are located below the euphotic zone. The 
bathyal zone extends down the continental slope from 200 m to 2000 or 
3000 m (the lower boundary is indefinite), and it occupies approximately 
16% of the submerged seafloor. The abyssal zone, extending from 2000 or 
3000 m to 6000 m, is by far the largest ecological región, encompassing 
almost 75% of submerged benthic habitats. This zone is also characterized 
by having a temperature of 4°C or less. The deepest areas of the sea are the 
trenches, extending downward from 6000 m to somewhat over 11 000 m 
depth; ecologically, this benthic habitat is referred to as the hadal zone 

(from ‘Hades’, the Greek mythological underworld). This last zone is the 
least well known because of its inaccessibility, and relatively few species 
have so far been described from it. 

The vast majority of larger benthic species live in depths less than 200 m, 
and there are many more species in shallow tropical waters than in shallow 
coid seas. Some of the different types of shallow-water and deep-sea benthic 
communities are considered in more detail in Chapter 8. 


A variety of marine plants attach to the seabed or live within sediments in 
shallow depths. All are restricted to the euphotic zone; that is, they are 
confined to intertidal and shallow subtidal regions. 

Certain intertidal marine communities are dominated by large angiosperms 
(flowering plants) that only flourish in sheltered regions, where accumulated 
sediments allow rooted plants to develop. These communities inelude 
tropical mangrove swamps, with a variety of salt-tolerant trees and shrubs 
(see Section 8.7); estuarine saltmarshes which are dominated by 
marshgrasses (see Section 8.5); and meadows of seagrasses which occur low 
in the interidal zone. All of these benthic macrophytes (large, visible plants) 
are highly productive, but they contain a large proportion of materials that 
are indigestible to most marine animáis. They therefore form large amounts 
of detritus, which may be exported by tidal currents into other marine areas, 
and this decomposing detritus contributes to the high productivity of 
Coastal waters. 

Marine macrophytes also inelude the conspicuous algae that are most 
abundant on rocky shores in températe zones. The algae (which are rootless) 
have developed anchoring structures called holdfasts. Among the important 
members of this group in terms of production are the long-stemmed kelps 
(brown algae) (Figure 7.1c and Figure 8.3) that anchor to rocky substrates in 
the subtidal zone (see Section 8.3). Some kelp (e.g. Macrocystis ) have 
extremely fast growth rates and form large underwater forests. Other types 
of seaweeds (Figure 7.1) inelude the common macrophytic algae (e.g. 

Fucus) that cover rocks in the intertidal zones; they can be very abundant, 
but their rate of production is usually only about half that of the kelps. Kelp 
and seaweeds may be a direct source of food for some herbivores, and they 
also form abundant detritus that is ultimately consumed by detritivores. As 
much as 30% of their production may be lost in exudates that contribute to 
the pool of DOM (see Section 5.2.1). 

Some green algae (e.g. Halimeda) and red algae (e.g. Lithothamnion) have 
the ability to incorpórate calcium carbonate in their tissues, which is a most 
effective defence against being eaten by herbivores. These hard coralline 
algae grow as encrustations over rocks or shells or coral reefs, and they can 
contribute materially to the formation of carbonate deposits. 

Epiphytic algae (e.g. Ectocarpus) grow on the surfaces of other larger plants 
(seaweeds, kelp or seagrasses). The epiphytes are generally thin-walled and 
filamentous, and therefore can be easily consumed by marine herbivores. 

The least obvious benthic producers are the unicellular algae that live on 
sand grains (epipsammic species), or that form mats on the surface of mud. 


Figure 7.1 Examples of seaweeds. 

(a) Enteromorpha (up to 500 mm); (b) Uíva (up 
to 250 mm); (c) Alaria (up to 2 m); 

(d) Chondrus (up to 150 mm); (e) Gigartina (up 
to 200 mm); (f) Delesseria (up to 250 mm); 

(g) Fucus vesiculosus (up to 1 m). (a) and (b) 
are green algae; (c) and (g) are brown algae; 
and (e) and (f) are red algae. 

These microphytes (microscopic plants) inelude motile pennate diatoms, 
blue-green algae, and dinoflagellates (see Section 3.1 for general 
descriptions). These organisms are often extremely abundant and despite 
their small size, they are an important source of primary production in 
shallow waters. Certain dinoflagellates have even taken up residence within 
the tissues of benthic animáis; the best known are the symbiotic algae of 
coráis that are described in Section 8.6. 

The rock-like reefs called stromatolites (Colour Píate 30) that are located in 
shallow waters off western Australia and around the Bahamas are of great 
interest from an evolutionary perspective. They are formed by mats of 
microphytic photosynthetic Cyanobacteria (see Section 3.1.3) that deposit 
calcium carbonate which builds up in successive layers at a rate of about 
0.5 mm per year. Although the present reefs began to form relatively 
recently in geologic time, the constituent Cyanobacteria are similar to 
microbes that flourished two billion years ago, and thus they represent one 
of the longest continuous biological lineages known. 

Different benthic plants characteristically occupy different tidal levels, and 
this zonation is partly determined by their different abilities to absorb 
particular wavelengths of light. The complete spectrum of visible light is 
available at the sea surface, but different wavelengths are quickly absorbed 
and scattered within the water column (Figure 2.4). Green algae (e.g. Ulvd) 
typically grow in shallow water, and their pigments absorb both long and 
short wavelengths (Figure 3.4a). Brown and red algae also contain green 
chlorophyll, but they have particular accessory pigments that mask its 
colour. Compared with green algae, brown algae (e.g. kelp, Fucus) are most 


abundant in somewhat deeper water; their main pigment, fucoxanthin, is 
more efficient at capturing blue-green light (Figure. 3.4b). Some red algae 
(e.g. Gigartina) are characteristically subtidal; their red pigments 
(phycoerythrin and phycocyanin) are also efficient at absorbing subsurface 
light that cannot be absorbed by chlorophyll a (Figure 3.4b). There are 
numerous exceptions to this depth-distribution pattern, however. For 
example, certain red algae (e.g. Porphyrá) may be found in the high 
intertidal zone, and some green algae (e.g. Ulva) may occupy lower regions. 
This is because other factors, such as resistance to wave action, tolerance to 
drying during tidal exposure, and selective grazing by herbivores also 
determine the position of plants in intertidal areas (see Section 8.2.1). In the 
sediment surface, blue light is absorbed first and red light penetrates the 
farthest; thus small algae growing within sand or mud may also show a 
different distribution from that described above. 

QUEST10N 7,1 Why are algae commonly grooped ín colour categories, such 
as 4 red' algae, *browiT algae, and 'green' algae? 


Production of small benthic plants can be measured by carbón or oxygen 
exchange, or estimated from chlorophyll concentration as described in 
Section 3.2.1. Production of benthic macrophytes, however, is commonly 
measured by harvesting and weighing the plants, with production being 
reported in terms of carbón per unit area. In températe areas where seasonal 
growth must be taken into account, harvesting is usually done at the period 
of máximum biomass; some allowance is made for estimating the biomass 
that has grown, but died and disappeared before and after the measurement. 
Growth rates of fronds of macrophytes can sometimes be obtained by 
punching holes in the plants; the hole will move away from the meristerm 
(growing región) of the plant, and growth can be expressed in terms of 
distance moved and incrase in the size of the hole. 

Attached plants have the advantage of being washed with turbulent water 
that brings a continual renewal of dissolved nutrients. Nutrient 
concentrations are often elevated in Coastal waters, and the rates of nutrient 
uptake are generally high in benthic plants. Usually the productivity per unit 
area of large attached algae is an order of magnitude greater than that of 
phytoplankton. Valúes for benthic productivity are given throughout 
Chapter 8, where specific types of communities are considered in more 
detail. Although benthic photosynthetic production exceeds that of 
phytoplankton in the water column in many Coastal areas, only a small 
fraction of the seafloor receives sufficient light to support attached plants. 

On a global scale, production by benthic plants accounts for less than 10% 
of the total primary production in the sea. 


Benthic animáis (or zoobenthos) are separated into two ecological categories 
based on where they live relative to the substrate. The infauna are those 
species that live wholly or partly within the substrate; this category includes 
many clams and worms (polychaetes) as well as other invertebrates 
(Figure 7.2). Infaunal species usually domínate communities in soft 


Figure 7.2 Representative ¡nfauna, showing their burrows and living positions. 

(a) Hydrobia, a snail; (b) burrow of Pygospio, a polychaete; (c) burrow of Corophium, an 
amphipod; (d) Arenicola , a polychaete; and the clams (e) Cardium, (f) Macoma, 

(g) Scrobicularia , and (h) Mya. 

substrates, and they are most diverse and abundant in subtidal regions. There 
are a few infaunal species in hard substrate communities as well, rock-boring 
clams being one example. The epifauna (Figure 73) are those animáis 
living on or attached to the seafloor; about 80% of the larger zoobenthos 
belong to this category. A few common examples of epifauna inelude coráis, 
barnacles, mussels, many starfish, and sponges. Epifauna are present on all 
substrate types, but they are particularly richly developed on hard substrates, 
and they are most abundant and diverse in rocky intertidal areas and coral 
reefs. A third category can be added to inelude those animáis that live in 
association with the seafloor but also swim temporarily above it; animáis 
such as prawns and crabs, or flatfish such as solé, form the epibenthos. 



Figure 7.3 Representative epifauna and epiflora. 


It is also convenient to classify benthic animáis into size categories. In this 
case, size is relative to the mesh size of sieves used to sepárate animáis from 
sediments. The following categories encompass all sizes of benthos: 

Macrofauna (or macrobenthos): those animáis retained by a 
1.0-mm-mesh sieve. These are the largest benthic animáis, including starfish, 
mussels, most clams, coráis, etc. 

Meiofauna (or meiobenthos): those animáis retained by a 
O.M.0-mm-mesh sieve. These are small animáis commonly found in sand or 
mud. The group ineludes very small molluscs, tiny worms, several small 
crustacean groups (including benthic copepods), as well as less familiar 
invertebrates (see Section 8.4.2 and Figure 8.5). 

Microfauna (or microbenthos): those animáis that are smaller than 
0.1 mm in dimensión. This smallest size category is largely made up of 
protozoans, especially ciliates (Figure 7.4). 


Benthic communities contain an extremely diverse assemblage of 
zoobenthos. Many of these marine species have no terrestrial or freshwater 
counterparts and are unfamiliar animáis. Some of the dominant types of 

Figure 7.4 Examples of microfauna: ciliate protozoans showing diversity of form. 


Table 7.1 Major taxonomic groups and representatives in marine benthic 



Common names/representatives 












hydroid polyps 



sea anemones; coráis 








ribbon worms 



polychaete worms 


beard worms 


vestimentiferan worms 


sipunculids (peanut worms) 


echiurids (spoon worms) 



acorn worms 



snails; nudibranchs 


clams; mussels 













brittle stars 


sea urchins; sand dollars 


sea cucumbers 


feather stars; sea lilies 


bryozoans (moss animáis) 


lamp shells 


(Class Crustácea) 




cyclopoids; harpacticoids 










crabs; lobsters; shrimp 



tunicates (sea squirts) 

animáis in benthic communities are listed in Table 7.1 and illustrated in 
Figures 7.5-7.7. They are described below, with particular attention being 
directed to their positions in benthic food webs. 

The best known of the benthic Protozoa are the foraminifera, whose 
planktonic relatives were described in Section 4.2. Several thousand benthic 
species are known, and they form a dominant element of the micro- and 
meiobenthos, particularly in deep-sea sediments. Although they are 
unicellular organisms, benthic forams are not necessarily small in size; some 
attain lengths of 25 mm. There are both epifaunal and infaunal species, and 
in general the various species feed on benthic diatoms and algal spores in 
shallow water, and on other protozoa, detritus, and bacteria in all depth 


zones. Newly discovered relatives, the Xenophyophoria (Figure 7.5a), are 
especially abundant in hadal zones. They are the largest of all protozoans, 
with diameters of up to 25 cm but only 1 mm thick. Their extended 
pseudopodia (to 12 cm long) form tangled masses on the seafloor, and these 
sticky structures probably collect organic matter from surface sediments. 
Ciliates (Figure 7.4) are important members of the microbenthic community; 
many are adapted to attach to sand grains or to live freely within the 
interstitial spaces of sediments. The fragility of ciliates has hampered 
sampling and ecological studies, but these protozoans are no doubt an 
important link in shallow water between the microflora (e.g. benthic 
diatoms) and larger animáis and, at all depths, between bacteria and 
deposit-feeding invertebrates. 

The most primitive multicellular animáis are the sponges (Figure 7.5b), 
which may constitute a large fraction of the macrobenthos in some marine 
regions. Known to exist from late Precambrian times (>600 million years 
ago), this ancient group now has roughly 10000 species, almost all of them 
marine. They are named for their porous nature (Phylum Porifera), and the 
many cavities of sponges provide protective refuges for myriads of small 
animáis such as worms and crustaceans. All sponges are sessile, that is, they 
are attached and immobile. Most filter-feed by producing currents that draw 

Figure 7.5 Representative benthic animáis: 

(a) unicellular xenophyophore; (b) sponge; 

(c) sea anemone; (d) nemertean; (e) polychaete; 
(f) flatworm; (g) enteropneust hemichordate; 

(h) echiurid; (i) sipunculid; and 
(j) pogonophoran. (All scales in mm.) 


suspended partióles through the sponge. The pores of the sponge act like a 
sieve, allowing only the smallest partióles to pass and be captured by special 
flagellated cells. Food consists largely of bacteria, nanoplankton, and small 
detrital partióles. The sponge skeleton is composed of calcium carbonate or 
siliceous spicules embedded in the body wall, or of spongin fibers. Because 
of their hard spicules, and reputedly because of their bad tasto, sponges have 
few predators, the exceptions being some coral reef fish and some snails and 
nudibranchs. They have both asexual and sexual reproduction, and are 
capable of regenerating from only fragments of a whole organism. 

The Phylum Cnidaria has many benthic representad ves in addition to the 
pelagic species described in Chapter 4. This group too has had a long 
evolutionary history, and there are now species living in most marine 
environments. Most of the bottom-dwelling species are epifaunal but a few 
exceptional species have adapted to live within sand or mud. Although there 
is considerable diversity within the group, all benthic cnidarians are 
characterized by having a radial symmetry, and all are suspensión feeders 
that capture prey using nematocyst-laden tentacles. Some species also trap 
very small particulate food in mucus secreted onto their oral surface. Benthic 
cnidarians are sessile animáis, although some sea anemones are capable of 
detaching from a substrate and swimming temporarily to escape starfish 
predators. Asexual as well as sexual reproduction is common in 
this phylum. 

Within the Phylum Cnidaria, the Class Hydrozoa ineludes the colonial 
hydroids (Figure 7.3), formed of unions of structurally and functionally 
different types of individuáis. Although they are usually small and 
inconspicuous, a large part of the marine growth attached to rocks, shells, 
and wharf pilings and usually called ‘seaweed’ is actually composed of 
hydrozoan colonies. Some hydroids produce free-swimming medusae as part 
of their life cycle, but in the majority of species the medusa remains attached 
to the parent, where it functions as a sexually reproducing individual. The 
much larger Class Anthozoa, with over 6000 species, ineludes sea anemones 
and a variety of coráis, as well as less familiar forms such as sea whips and 
sea fans. Sea anemones (Figure 7.5c) are common residents of intertidal and 
subtidal communities, but are also found at over 10000 m depth; they are 
solitary animáis, ranging in diameter from about 1 cm to more than 1 m. 
Included among the Anthozoa are a variety of taxonomically different forms 
called ‘coráis’; the important subgroup of stony coráis that form massive 
reefs in tropical regions is considered in more detail in Section 8.6. 

Benthic worms belong to a number of different phyla. The threadlike 
nematodes (Phylum Nematoda) constitute one of the most numerous and 
widespread groups of marine (and terrestrial) animáis, although most of the 
species are inconspicuous inhabitants of soft sediments. A single square 
metre of bottom mud off the Dutch coast was reported to contain about 
4 500000 individual meiobenthic nematodes. Taxonomic problems have 
hampered ecological research on this abundant group, but it appears that 
there is a wide diversity in feeding types with some species being 
carnivorous, others feeding on plants, or on decaying material and associated 
microfauna. The Phylum Nemertea (Figure 7.5d) encompasses about 600 
species of elongated worms, all characterized by having a long eversible 
proboscis that is used to capture food. Nemerteans are more abundant in 
températe seas than in tropical areas, and they are more common in shallow 
zones. Free-living flatworms (Phylum Platyhelminthes) (Figure 7.5f) reside 

in sand or mud, under stones and shells, or on seaweeds, but they are seldom 
present in large numbers. Sipunculids (Phylum Sipuncula) (Figure 7.5i), 
also called peanut worms, are unsegmented worms ranging in length from 
about 2 mm to more than 0.5 m. Many of the 250 or so species burrow into 
sand or mud, using movements of their large proboscis to forcé their way 
through the sediments; others inhabit rock or coral crevices, or even empty 
snail shells. They are mostly deposit feeders. Echiurids (Phylum Echiura) 
(Figure 7.5h) are somewhat similar to sipunculids in size and general habit. 
Most species use their large nonretractible proboscis to forage for food 
contained in sediments. Although some species occur intertidally, most are 
found only in very deep water habitats. The majority of deep-sea echiurid 
species have dwarf parasitic males attached to the female, a mode of 
reproduction that is reminiscent of that of the deep-sea angler-fish (see 
Section 6.6). 

More than 10000 species belonging to the Phylum Annelida, Class 
Polychaeta, make up the largest and most diverse group of marine worms. 
Polychaetes (Figure 7.5e) are the segmented worms with múltiple 
appendages called parapodia. Size ranges from a few millimetres to 3 m in 
length. Ecoiogically, polychaetes can be separated into those that move 
actively over the seafloor or burrow into sand and mud, and those that 
inhabit permanent tubes or burrows. Most crawling species and some of the 
active burrowers are camivorous and feed on various small invertebrates that 
are captured with the predator’s jaws. Some polychaetes also use their jaws 
to tear off pieces of algae. Many burrowers and some tube dwellers are 
deposit feeders that consume sand or mud directly by mouth. Other 
deposit-feeding species have developed special tentacle-like structures that 
extend onto or into the substrate; sediment particles adhere to mucous 
secretions on the surface of these structures, and this material is then 
conveyed to the mouth by cilia. As well, many of the sedentary species are 
filter feeders, using special head appendages to collect plankton and 
suspended detritus. This group, with both epifaunal and infaunal species, 
frequently forms a large fraction of the benthic biomass in many habitats. 

The pogonophora (Figure 7.5j) are regarded as specialized annelids by 
some workers, or as a sepárate phylum by others. These sessile worms are 
most abundant in deeper areas, occurring down to 10000 m. They secrete 
long leathery tubes that are attached to hard substrates. A cluster of tentacles 
projects from the tube, and the common ñame of ‘beard worms’ is derived 
from this feature. The pogonophora are highly unusual in lacking a mouth or 
gut, but they share these features with the vestimentiferan worms that are 
described in Section 8.9 and shown in Colour Píate 39. Both groups depend 
on symbiotic chemosynthetic bacteria for their nutritional requirements, 
although pogonophorans may also utilize dissolved organic substances 
absorbed through their tentacles. 

The Phylum Hemichordata ineludes the enteropneusts (Figure 7.5g), or 
acorn worms, which occur intertidally as well as at deep-sea hydrothermal 
vents (Section 8.9) and in trenches (Section 8.8). The largest species 
reportedly attains lengths of over 1.5 m, but most are much smaller. Many 
live in burrows in mud and sand, others move sluggishly over the sediment 
surface, or form entanglements on firm substrates. Burrowing forms use their 
proboscis to plough through the sediments, and most ingest sand or mud 
from which the organic matter is digested. The amount of substrate 
consumed is indicated by the large accumulations of faecal castings that 


accumulate at the posterior opening of the burrow. Nonburrowing species 
and some burrowers are suspensión feeders; plankton and detritus stick to 
the mucus-covered proboscis and then are transponed in ciliated grooves to 
the mouth. Enteropneusts are difficult to collect because of their fragility, but 
deep-sea cameras have recorded their trails and tangled masses on the 

QüESTION 7.2 Tab 1 e 7 J lists seven phyla o f d i f'fere n t i y pes of m ari ne worm s. 
Whui features do these animáis share lo classify as a 'worm\ and why do 
yon think they are such a sticcessful marine benthic group? 

Members of the Phylum Mollusca inelude over 50000 marine species, 
among them the familiar snails and related nudibranchs or sea slugs (Class 
Gastropoda), and the bivalved clams and mussels (Class Bivalvia). This 
phylum also ineludes the flattened chitons (Class Polyplacophora) (see 
Figure 8.2), with a shell divided into eight plates. Less well-known members 
are the burrowing scaphopods (Class Scaphopoda) (Figure 7.7a) with 
tusk-shaped shells, and the wormlike, shell-less aplacophorans (Class 
Aplacophora) found within sediments. Most of the octopus species (Class 
Cephalopoda) are also essentially benthic species, although they are capable 
of swimming. The great diversity in this phylum is expressed in the faets 
that molluscs inhabit all depths of the sea, are found both on and within 
sediments, have representative species in all trophic levels, and are present 
in all benthic communities. 

Echinoderms (Phylum Echinodermata) (Figure 7.6) are exclusively marine 
animáis. Although differing in external appearance, all echinoderms are 
characterized by having radial symmetry, with the body divided into five 
parts around a central axis; a skeleton composed of calcareous plates; and 
tube feet. The approximately 5600 species are divided into five classes. The 
Class Asteroidea ineludes roughly 2000 species of starfish (or seastars) (see 
Figure 8.2) whose habitats range from intertidal zones to about 7000 m. 
Many starfish are carnivorous, and they may have considerable ecological 
impact on cultivated shellfish beds as well as in natural habitats. Other 
starfish species are deposit feeders or, more rarely, suspensión feeders. The 
Class Ophiuroidea comprises almost 2000 species of long-armed brittle 
stars (Figure 7.6c) and basket stars. Deep-sea photographs often show 
ophiuroids carpeting the seafloor, where they feed on deposited sediments, 
on small dead or living animáis, or on suspended organic material. Some 
800 species of spiny sea urchins (Figure 7.6d) and flattened sand dollars are 
placed in the Class Echinoidea. Urchins are conspicuous members of the 
macrobenthos of rocky shores, kelp beds, and coral reefs; they use a special 
chewing apparatus to feed on all types of organic material, but most 
shallow-water species are regarded as basically herbivorous and deep-sea 
species (to about 7000 m) are considered to be deposit feeders. Sand dollars 
are inconspicuous, but often numerous, infaunal species; some are capable of 
suspensión feeding as well as deposit feeding. The Class Holothuroidea 
(with 500 species) ineludes the elongated sea cucumbers (Figure 7.6b), 
named because of their resemblance to the vegetable. The epibenthic species 
of holothurians can be either deposit or suspensión feeders; infaunal species 
swallow the sand or mud in which they live. Although also found in shallow 
waters, the greatest number of abyssal echinoderms are sea cucumbers, and 
they feature prominently in deep-sea photographs. The Class Crinoidea is 
the most ancient echinoderm group, and presently ineludes about 650 species 
of animáis known commonly as feather stars and sea filies. Feather stars 


Figure 7.6 Representative benthic echinoderms: (a) feather star (crinoid); (b) sea cucumber 
(holothurian); (c) brittle star (ophiuroid); and (d) sea urchin (echinoid). (All scales in mm.) 

(Figure 7.6a) live mostly in depths above 1500 m, and although they often 
cling to the seafloor, they are mobile animáis that are capable of crawling as 
well as of swimming temporarily. Sea lilies are attached by stalks to the 
seabed, and they are typically deep-sea inhabitants that are most abundant 
between 3000 m and 6000 m. All crinoids are considered to be suspensión 

Bryozoa (Figure 7.7b), or moss animáis, belong to the Phylum Ectoprocta. 
Like the hydroids, bryozoans are colonial and sessile animáis that form 
inconspicuous encrustations or seaweed-like growths on intertidal rocks, 
shells, or artificial surfaces. A few species have also been recorded from 
depths of more than 8000 m. Each individual in a colony is small (usually 
<0.5 mm long) and, in the majority of species, is largely encased in an 
extemal calcium carbonate skeleton. A special food-trapping device, called a 
lophophore, consists of numerous ciliated tentacles, and it can be projected 
into the overlying water to capture small planktonic food or suspended 
detritus. Despite including almost 4000 marine species, the group has 
received little attention from ecologists. 

Brachiopods (Figure 7.7c) constitute a phylum (Brachiopoda) of somewhat 
less than 300 marine species that superficially resemble molluscs in having a 


Figure 7.7 Representative benthic animáis: 

(a) scaphopod; (b) bryozoan colony; (c) stalked 
brachiopod; (d) two tunicates; (e) tanaid; 

(f) isopod; and (g) a horseshoe crab (not 
related to true crabs). (Al! scales in mm.) 

bivalved calcareous shell (5-80 mm in diameter), though their fundamental 
body-plan is quite different. Most live in the upper 200 m, and most are 
cemented to a hard substrate. However, some of the more common 
brachiopods (e.g. Ungula) live in vertical burrows in sand or mud, and some 
have been collected from 5500 m. Like the bryozoans, brachiopods possess a 
lophophore that is employed in suspensión feeding. 

Tunicates (or ascidians) (Figure 7.7d) are benthic relatives of the pelagic 
larvaceans and salps, described in Section 4.2. These sessile barrel-shaped 
animáis belong to the Phylum Chordata, Ciass Ascidiacea. Most of the 
common tunicates are solitary organisms, but there are also many colonial 
species that develop by asexual budding. Ascidians are commonly found in 
intertidal waters, attached to rocks, shells, wharves, or other firm substrates, 
but they also inhabit depths to at least 8000 m. The free end of a tunicate 
has two siphons that provide passage for a current of water drawn through 
the animal by cilia. Suspended partióles are removed from the water by a 
sheet of secreted mucus, and entangled food is conveyed to the gut by cilia. 
Filtered water is forcibly expelled from the animal’s excurrent siphon, giving 
rise to the common ñame of ‘sea squirtk Deep-sea forms exhibit 
modifications in the feeding apparatus, and they are thought to feed on 
suspended sediments, or even directly on meiobenthos. In general, however, 
the suspension-feeding mechanism of these benthic tunicates is similar to 
that employed by their pelagic relatives, the salps. Although ascidians are 
generally not major elements of benthic communities, they are capable of 


removing significant amounts of plankton or suspended material from water 
in proximity to the seafloor; a single tunicate only a few centimetres long 
can filter about 170 litres of water daily. 

The segmented Crustácea are well represented on the seafloor. Meiobenthic 
species inelude ostracods and the cyclopoid and harpacticoid copepods, all 
of which were described briefly in Section 4.2. Harpacticoids are an 
especially abundant group whose members crawl over or burrow through 
soft sediments. Also included in this size category are the tanaids 
(Figure 7.7e). These small (usually <2 mm long) crustaceans have slender, 
more or less cylindrical bodies, and they are burrowers or tube dwellers 
present at all depth levels down to at least 8000 m. Little is known about the 
biology of these 350 species. 

Common macrobenthic crustaceans inelude the isopods (Figure 7.7f) and 
amphipods. Isopods usually have a flattened body of 5-15 mm in length; 
deep-sea species are generally larger, however, with one genus reaching 
40 cm. Isopods are often observed running rapidly over rocks in the 
intertidal zone, but some species are burrowers, including one group that 
tunnels into wood. The majority of the 4000 marine species are omnivorous 
scavengers. Amphipods are closely related to isopods, but differ in that most 
have a laterally compressed body (see Figure 4.10 k for planktonic species). 
They range from a few mm to about 30 cm in size, with the largest species 
occurring in the deep sea. Depending on the species, amphipods are capable 
of crawling or burrowing, but many of the bottom-dwellers are also capable 
of swimming, even if infrequently. Many of the species construct temporary 
or permanent burrows or tubes. The depth distribution in this group ranges 
from the semi-terrestrial beach fleas living near the high tide level to species 
living in hadal trenches. Most amphipods are detritivores or scavengers, but 
a few are specialized filter feeders. 

Barnacles (Cirripedia) (see Colour Píate 31) are familiar marine animáis and 
the only sessile crustaceans. There are about 800 species, including a large 
number that are parasitic in other marine invertebrates. These shrimplike 
animáis live within an extemal covering of calcareous plates. Some attach 
directly to substrates, others are stalked. The more familiar barnacles form 
crowded aggregations in rocky intertidal regions, but some species have 
become specially adapted to attach to mobile surfaces and live on the bodies 
of whales, sharks, sea snakes, manatees, fish, or crabs. Although most 
common in shallower depths, some species are present to at least 7000 m. 
Free-living (i.e. nonparasitic) barnacles feed by rhythmically sweeping their 
feathery appendages through the surrounding water. These animáis often foul 
the bottoms of ships and the surfaces of buoys and wharf pilings. They have 
been the subjeet of extensive ecological studies since Charles Darwin’s 
elassie taxonomic monograph on this group. 

Benthic decapod crustaceans inelude the familiar crabs, lobsters, and 
shrimp, and the group has both epifaunal and infaunal representatives. 
Decapods show their greatest diversity in shallower waters, but a few species 
live at depths of 5000-6000 m. The group ineludes predators, omnivores, 
and scavengers. Some are filter feeders (e.g. burrowing mud shrimp and 
mole crabs), but detritus rather than plankton is often the dominant food. 
Many species in this group are economically important as human food, and 
those species along with molluscs make up the shellfish industry. 


There are a few other phyla of marine animáis that have not been considered 
here, either because they consist of only a few species or because they are 
not usually abundant in benthic communities. In some cases, we still lack an 
understanding of the ecological role of certain obscure animal groups simply 
because present sampling gear does not adequately collect them. Many new 
species (as well as new families) have been discovered within the last 
25 years, primarily in the deep sea (see Sections 8.8 and 8.9), and this list 
can be expected to continué to grow. 

QUE5TIQN 7.3 From Information in the previous section* what are some of 
the mechan i sois used by benthic animáis for defence against preda t ion? 


Benthic animáis can be collected by a variety of sampling gear. In shallow 
waters less than about 30 m, direct observation, counting, and collection by 
scuba diving are effective techniques that permit a diver to sample any type 
of substrate and provide a greater appreciation of the natural conditions, 
including patchy distribution of the macrobenthos. In depths beyond safe 
diving limits, grabs or box corers are commonly used to sample soft 
sediment communities. Ideally, these devices remove a quantitative bottom 
sample from which the resident animáis can be screened out on a series of 
sieves. Hydraulic cores of undisturbed samples can be obtained but because 
these are small, they are used only for quantitative sampling of meio- or 
microbenthos. Dredges are containers equipped with sturdy bags of fairly 
wide mesh that are pulled over the seafloor; they are designed to scoop up 
benthic animáis, but they do not generally provide quantitative samples. 
Other types of sampling devices are mentioned in Chapter 8 in connection 
with special environments. 

Standing stock of benthos is given in terms of numbers of animáis per unit 
area (usually m^ 2 ). Benthic biomass is expressed in terms of g m -2 ; ideally, 
these data are presented in grammes of carbón or grammes of dry weight, 
but they may be given in grammes of wet weight (for conversions, see 
Appendix 2). Biomass changes constantly, and measures of secondary 
productivity take this into account. Biomass increases through growth and 
reproduction of the individuáis in a population or community; existing 
biomass is removed by predation or other sources of mortality. Under certain 
circumstances, these changes can be monitored over successive intervals, and 
production can be determined from the methods given in Section 5.3.1 (see 
especially equations 5.3, 5.4, and 5.5). 

Age classes may be distinguishable in populations that breed during a 
well-defined time period, or from age-size relationships, or in species that 
have age-related features such as growth rings on shells. If the biomass of 
each age class is determined and changes in biomass are followed by 
successive sampling, it is possible to calcúlate production during the interval 
between samples. Table 7.2 presents production data collected over a period 
of 616 days from a population of infaunal clams in the North Sea. In the 
first interval of 50 days, mortality was very high but the mean weight of the 
surviving individuáis increased by about 4 mg; this resulted in a decrease in 
biomass, but a net production of 317 mg m -2 day~ l . Completing the 
calculations in the table will show that proportionally more biomass was 
produced, but then lost from the population due to predation or other 
sources, during the first two intervals than later in the study. 


Table 7.2 Production data from a population of the clam Mactra in the North Sea. 











(no. m -2 ) 


(mg m -2 ) 

(mg m -2 day” 1 ) 

(mg m -2 day -1 ) 




















QUESTION 7,4 ín Table 7.2. biomass has becn calculated only tbr the lirst two 
age classes. and loss of biomass and net production only for the first interval 
of 50 days. Complete the rest of the table using equation 5,3 to calcúlate 
biomass. and equation 5.4 to calcúlate biomass losses and net produclUm. 
(Refer to Section 5.3.1 for help if necessary.) 

If it is not possible to follow a single cohort of animáis and there are 
numerous overlapping age classes, benthic secondary productivity becomes 
more difficult to assess directly. In such situations, laboratory studies can be 
used, if feasible, to determine typical growth rates of each arbitrary size 
class. The population is then divided into component size classes regardless 
of age, and production is calculated by multiplying the numbers present in 
each size class by the growth rate of that class. 


The numbers and types of organisms making up any particular benthic 
community are determined by a variety of physical and biological factors. 
Among physical factors, sediment type is of paramount importance in 
dictating the type of community that can become established in a particular 
locality. The relative proportions of epifauna and infauna will vary according 
to whether the seafloor surface is composed of rock, mud, sand, or some 
combination of these types. At the same time, however, the activities of the 
organisms themselves will alter the substrate. Burrowing and feeding 
activities of the infauna continually disturb and rework soft sediments (a 
process called bioturbation), and sessile organisms that attach to hard 
surfaces alter the topography of their environment. In shallow Coastal waters, 
tidal levels, degree of wave action, and salinity and temperature variations 
will influence the composition and relative abundance of benthic species. 

Biological determinants of benthic community structure inelude competition, 
predation, and the type of development leading to recruitment into the adult 
population. As in pelagic communities (Section 5.2), competition among 
individuáis or among species for limited resources (e.g. food) may affect 
population densities and relative abundance of species. Competition for 
limited space in hard-substrate communities may influence the distribution 
pattems of sessile zoobenthos. Selective feeding by predators can alter the 
outeome of competition between prey species, and predation may affect total 
species diversity in benthic communities. The effeets of competition and 


predation in specific types of communities are discussed in more detail in 
Chapter 8. 

Community structure also can be influenced by the type of development that 
is exhibited by dominant species. Benthic animáis may have direct 
development, in which there is no free-swimming larval stage and the young 
emerge as miniature adults, or more commonly, they may have indirect 
development with the production of meroplanktonic larvae (see Section 4.3). 
Two types of meroplanktonic larvae are distinguished. Lecithotrophic 
larvae hatch from relatively large eggs that are produced in small numbers 
by the parent. They spend a very brief time in the water column (hours to 
2 weeks) and usually do not feed on planktonic food, depending instead on 
yolk in the egg for their growth and development. Planktotrophic larvae 
hatch from small eggs produced in large numbers; they rely on 
phytoplankton or bacteria for nourishment and remain in the pelagic 
environment for several weeks or months. 

Lecithotrophic larvae, which spend a very short duration in the plankton, do 
not usually drift far from the adult population which produced them, but 
planktotrophic larvae remain in the water column for longer times and can 
be transported by currents over long distances. Consequently, planktotrophic 
larvae in particular are a means of dispersal for species that are immobile or 
slow-moving as adults. Both larval types are vulnerable to pelagic predators 
while they are planktonic, and planktotrophic larvae especially may be 
carried away by currents from suitable benthic substrates upon which they 
could successfully settle. Lack of abundant planktonic food may also 
contribute to the higher mortality rates of planktotrophic larvae. As 
meroplanktonic larvae develop and grow, they become photonegative and 
begin to move deeper in the water column, toward the seafloor. Many, if not 
all, pelagic larvae actively search for a suitable substrate before selecting one 
on which to settle and metamorphose into the adult form. Larvae seeking 
settlement sites rely on a variety of cues such as substrate type, suitable 
food, light intensity and, in many cases, on the presence of adults of their 
own species. 

The relative success of larval recruitment into adult populations will 
influence community structure in terms of relative numbers and biomass of 
the resident species. Species that produce planktotrophic larvae often have 
variable recruitment success; in successful years, many larvae may survive to 
settle in the adult population, but in other years, larval mortality may be high 
and recruitment low. In comparison, populations of species with 
lecithotrophic larvae (or those with direct development) tend to have low but 
constant biomass over long periods. 

QUESTiON 7.5 Benthic in vertebrales with direct development (no pelagic 
larval stage) may neveitheless be widely distributed, Can you think of any 
way(s) in which such species could disperse to new geographic local i lies? 


1 The benthic environment is divided into a number of distinctive 
ecological zones based on depth, seafloor topography, and vertical gradients 
of physical parameters. These are the supralittoral, littoral, sublittoral, 
bathyal, abyssal, and hadal zones. 

2 Benthic plants inelude macrophytic angiosperms like mangrove trees, 
marshgrasses, and seagrasses. Macrophytic algae inelude green, red, and 
brown seaweeds, and the long-stemmed kelps, a type of brown algae. 
Microphytic algae inelude benthic species of diatoms, Cyanobacteria, and 

3 The number of phyla and the number of species of benthic animáis 
exceeds those of pelagic species, at least partly because of the greater 
physical variety of benthic habitats. 

4 Benthic animáis are separated into infaunal and epifaunal species, 
depending upon whether they live within sediments or on the surface of the 
seafloor, respectively. Size categories of the zoobenthos consist of the larger 
macrofauna (>1.0 mm), the small meiofauna which is characteristically 
found in sand and mud, and the microfauna which is made up mostly of 

5 Benthic primary productivity is measured by a variety of methods 
including the carbón-14 method for microphytic species, and harvesting and 
weight measurements of macrophytic plants. Methods of estimating benthic 
secondary production are similar to those employed for pelagic animáis and 
described in Chapter 5. 

6 The numbers and types of species making up any particular benthic 
community are determined by a variety of physical and biological factors. In 
shallow Coastal communities, the types of species present and their relative 
abundance will be partly determined by tidal levels and degree of exposure 
to air, wave action, and range of salinity and temperature. At all depths, the 
type of sediment (e.g. sand, rock, mud) will dictate the relative proportions 
of epifauna and infauna. Biological factors that influence benthic community 
structure inelude competition for limited resources (e.g. food, space), 
predation, and type of development. 

7 Benthic animáis may have direct development, in which there is no 
free-swimming larval stage, or they may produce pelagic planktotrophic or 
lecithotrophic larvae. Planktotrophic larvae are relatively small and are 
produced in large numbers; they must feed on planktonic food, and they 
remain in the water column for several weeks or months. Although they are 
a means of dispersal for the species, planktotrophic larvae have high rates of 
mortality, and the adult populations have variable rates of recruitment from 
year to year. Lecithotrophic larvae hatch from relatively large eggs that 
contain large amounts of nutritive material and that are produced in small 
numbers; these larvae do not remain planktonic for long, and they do not 
feed while in the water column. Compared with planktotrophic larvae, 
mortality rates are lower for lecithotrophic larvae, and species with this type 
of development tend to have low but constant biomass because recruitment 
is less variable. 

Now try the following questions to consolídate your understanding of this 
Chapter . 

QUESTIQN 7,6 Compare Tabíe 7.1 with Table 4J (a) How many phvlu are 
represented in the plankton and in the bembos? (b) Why is there a difference 
in the numbers oí phyla in these major marine envrronmems? 


QUESTIOH 7.7 Wüuld you cortsider benthic species tbat produce leeithotrophi 
larvae to ha ve predominantly r- or K -selection characteristícs? Explain the 
bases for your answer (Refer to Section 1.3.1 and Table l.L) 

0UEST10N 7,8 Slromatolites (see Section 7 A) in Hainelin Pool. Western 
Australia, reach a máximum height of 1.5 m. Assuming no erosión or 
change in sea level, how oíd are diese structures? 



This chapter considers different types of benthic communities, ranging from 
the highest intertidal levels to the deepest trenches. Although these benthic 
habitats are treated separately here, they are all coupled in a dynamic fashion 
with the overlying pelagic environment. 

In shallow water, both phytoplankton and benthic plants contribute to the 
primary production of benthic communities. Phytoplankton and zooplankton 
are consumed by shallow-living filter feeders. Conversely, exudates and 
detritus from attached benthic plants supply necessary nutrients for the 
phytoplankton and planktonic bacteria, and bottom currents may cause 
resuspension of sediments, making benthic microphytes and bacteria-covered 
sediment particles available as food sources for zooplankton. Some fish and 
some marine mammals rely on shallow-water benthos for food. 

The great majority of benthic communities, however, are located in the 
aphotic zone, and most are entirely dependent on organic matter that is 
photosynthetically produced in the euphotic pelagic zone. The only 
exceptions are certain communities in which the food chain begins with 
chemosynthetic production by bacteria (see Section 8.9). Part of the organic 
matter that sinks or is transported from the surface waters is the food source 
that supplies deep-water benthic communities. Sinking organic and inorganic 
particles also form the sediments in which the benthos live. Decomposition 
processes tend to take place in deep water or on the seafloor, and the 
nutrients that are released are eventually returned to the surface where they 
are used by the phytoplankton. 

Marine organisms may exploit both the benthic and pelagic environments 
during different stages of their Uves. Many invertebrates are benthic as 
adults, but disperse by producing planktonic larvae. Conversely, some 
planktonic organisms produce resting stages (spores, cysts, or eggs) that sink 
into sediments where they remain dormant until favourable water conditions 
cause them to ‘hatch’ into swimming or drifting stages. 

The concept of benthic-pelagic coupling recognizes the many interactions 
between these two vast environments, and attempts to intégrate the ecologies 
of the seafloor and the water column. 


The terms littoral and intertidal are synonyms for that Coastal area which is 
periodically exposed to air by falling tides and submerged by rising water 
levels. Included in this general area are a variety of distinctive ecosystems 
such as rocky shores, sandy beaches, and mudflats, each supporting specially 
adapted assemblages of species. Rocky intertidal areas support a 
preponderance of epifauna, whereas the soft-substrate sand and mud 
communities have higher proportions of infaunal species. The intertidal 
regions mark the transition from land to sea, and although they make up 
only a very small part of the total world ocean, they support rich and diverse 
communities of marine plants and invertebrates as well as birds and inshore 
species of fish. Even some land mammals (e.g. mink, skunks, and raccoons) 


visit this area to feed on easily accessible shellfish, and many shorebirds 
depend on the rich food supply to be found in these habitats. 

8.1.1 TIDES 

Tides are the periodic rise and fall of sea level over a given time interval, 
and they are caused by the interaction between the gravitational attraction of 
the Moon and Sun on the Earth, and the centrifugal forcé resulting from the 
rotation of the Earth and Moon. On most coasts, semidiurnal tides result in 
the intertidal zone being exposed and covered by water twice each day. 
However, because of certain physical conditions, there is only one tide per 
day (a diurnal tide) in some regions such as the Gulf of México. 

Tidal range is greatest during spring tides which occur twice each month 
when the Earth, Sun, and Moon are aligned. At the other extreme, tidal 
range is minimal during the neap tides which occur at the first and the third 
quarters of the Moon, when these planetary bodies are not in alignment. The 
high water mark is the greatest height to which the tide rises on any day, 
and the low water mark refers to the lowest point to which the tide drops. 

The extent of the littoral zone in any particular locality is governed by the 
slope of the shoreline and by local tidal ranges, which are partly determined 
by the configuration of coastlines. Tidal range varies from barely perceptible 
in places such as Tahiti and the Baltic Sea, to as much as 15 m in the Bay of 
Fundy in eastem Cañada. 


The littoral regions experience the greatest variations in environmental 
conditions of any marine areas. Here, organisms are periodically exposed to 
air, and they encounter wide fluctuations in temperature and salinity. Rainfall 
and land runoff both contribute to lowering salinity. In coid climates, 
intertidal organisms are subjected to ice formation and ice scouring. In 
addition, many intertidal regions are exposed to heavy wave action and 
current motion. 

Intertidal plants and animáis show a variety of special adaptations to the 
changing conditions of their environment. Whereas inhabitants of sand and 
mud tend to burrow into the soft substrates to escape desiccation, 
temperature and salinity extremes, and wave action, organisms living on 
rocky shores exhibit more diverse adaptations to these environmental 
features. Rocky-shore species of bivalved molluscs (e.g. clams, mussels) and 
barnacles cióse their shells tightly during emersión, enabling them to retain 
moisture around the gills and thus preventing desiccation as well as exposure 
to freshwater. Many snails also retreat into their shells, sealing the shell 
aperture with a horny or calcareous operculum on the foot. On the other 
hand, many benthic plants and some of the intertidal animáis have no 
particular mechanisms to avoid water loss. Algae like Fucus and 
Enteromorpha , for example, tolérate as much as 60-90% loss of water from 
their tissues. 

Shells, or other types of rigid exoskeletons like those of sea urchins, also 
protect animáis from mechanical injury in areas where wave action can be 
severe. In some sea urchins and molluscs, the shell is much thicker in 
populations exposed to heavy wave action than in populations which are 

sheltered. Strong attachment to rock surfaces or other firm substrates 
prevenís plañís and animáis from being washed away by waves and currents. 
Beníhic algae aííach lo rocks by special holdfasls. Barnacles, oyslers, some 
lubeworms, and lunicaíes secrele cemeníing subslances for firm attachment. 
Mussels secrele lough elaslic byssal íhreads from a special gland in íhe foot, 
and íhese secure íheir posilions. The broad, flallened fool of limpets, 
abalones and chitons provides a suction-like attachment, and their low, 
streamlined profiles also help to resist wave impact. Certain animáis (e.g. 
some sea urchins and rock-boring clams) are equipped to bore into hard 
surfaces by mechanical abrasión, Chemical secretions, or both. Many of the 
more mobile intertidal animáis, like crabs and isopodes, seek out rock 
crevices where the wave action is reduced; this sheltering behaviour also 
permits them to remain in moist refuges at low tide. Rock pools form similar 
refuges for animáis such as starfish, crabs, and small intertidal fish, all of 
which avoid desiccation by remaining in these pools at low tide. 


Much is known about the inhabitants and ecology of rocky shores compared 
with other marine habitats. The accessibility of these densely-populated 
marine communities has permitted researchers to make long-term direct 
observations, and to conduct in situ experiments on factors determining 
community structure. 


A striking characteristic of all rocky shores is that the resident plants and 
animáis are grouped in distinctive bands, with some species living high in 
the intertidal zone and others being grouped at lower tidal levels. This 
vertical zonation of species applies to all rocky intertidal communities, 
although the specific pattern of zonation and the species composition of the 
zones varíes according to geographic location, tidal range, and whether sites 
are exposed to severe wave action or are protected. Zonation is largely based 
on sessile species, like algae, barnacles and mussels, although some mobile 
animáis also tend to be zoned but with less sharp demarcation. In general, 
many of the larger motile animáis move with the tides and often remain in a 
relatively constant water depth, or retreat to rock pools at low tide. 

On rocky shores, the supralittoral zone (see Chapter 7) is inhabited by 
encrusting black lichens (which are combinations of algae and fungi) and 
blue-green algae, certain species of Littorina (periwinkles) that graze on the 
vegetation, and relatively large (3-4 cm long) isopods {Ligia). Primitive 
insects (e.g. Machilis) may also be present. 

Just below the supralittoral zone, periwinkles (.Littorina species) are usually 
found in extraordinarily dense populations, with numbers ranging from 
several hundred to 10000 snails per square metre. Lower in the intertidal, 
barnacles form a sharply demarcated belt, and these crustaceans may also 
have densities of thousands per m 2 . In many localities, mussels crowd 
together in dense aggregations below the barnacle zone. There is intense 
competition for the limited space among the attached algae and sessile 
animáis as shown in Colour Píate 31. 


Gregariousness is an adaptive feature in many of these intertidal species. By 
crowding together, periwinkles create microhabitats in which more moisture 
is retained during exposure at low tides. They reproduce by internal 
fertilization, and crowding also increases their chances of finding mates. 
Mussels freely release gametes into the sea, where fertilization occurs; in 
this case, gregariousness increases synchrony of spawning among many 
individuáis. Barnacles are hermaphrodites that cross-fertilize, and high 
population densities are necessary for reproduction. The penis of a barnacle 
can only reach about twice as far as the diameter of its exoskeleton, so it is 
essential that the animáis be in cióse proximity. 

Intertidal zonation of organisms is not determined simply by tidal levels, but 
results from a variety of physical and biological causes. The upper limit of a 
particular zone is often determined by physical factors and the ability of 
particular plants and animáis to deal with exposure to air and with 
temperature and salinity variability. The upper limit of any one species may 
also be set by biological factors such as the absence of suitable food, or 
grazing or predation pressure. The lower limit of a particular zone is usually 
determined only by biological factors. 

How can the causes of vertical zonation be determined? 

Sessile animáis, like barnacles and mussels, make ideal subjects for studies 
of vertical zonation because the same individuáis can be monitored over 
long periods. Photographs taken at successive intervals record the size and 
position of individuáis and enable researchers to follow growth, interactions 
with neighbouring individuáis, and death. The accessibility of the intertidal 
zone also makes experimental manipulation possible, and individual animáis 
may be removed from sites, or whole rocks with attached organisms may be 
relocated to different tidal levels. It is also possible to exelude predators by 
enclosing study populations within wire cages. The results of such 
techniques are described below. 

Barnacle populations on rocky shores in Scotland are composed of two 
species (Figure 8.1). Adults of the small Chthamalus montagui form a 
distinct band high on the shore, above the mean high water mark of neap 
tides, with only a few adults being found down to the mean tide level. The 
larger Balanus balanoides has a much wider distribution for both adults and 
larvae, extending above and below the mean neap tide marks. Long-term 
observations indicated that the larvae of Chthamalus actually settle 
throughout most of the Balanus zone, but only survive to adulthood in the 
upper areas. This is because young Chthamalus are eliminated from areas 
below the mean high water neap mark by competition for restricted 
attachment space with the faster growing Balanus . Balanus either overgrows, 
undercuts, or crushes Chthamalus. The observation that the lower limits of 
Chthamalus zonation are dictated by biological competition was confirmed 
by experimental work in which all newly settled Balanus larvae were 
removed from rocks containing young Chthamalus ; in such studies, 
Chthamalus survived well at all tidal levels. On the other hand, the upper 
zonation levels for both barnacle species are determined by physical factors, 
with Chthamalus being more tolerant to heat and/or desiccation than 
Balanus . Further, intraspecific competition may be an important mortality 
factor for Balanus ; if larval recruitment is high, the growing barnacles begin 
to compete for space, and the younger or slower-growing individuáis of the 
species are eliminated. 


Balanus óatanoiúes 

Chtfiamtus montagtsi 

mear high 
water spring 

mean higfi 
water neap 

mean tide 

mean low 
water neap 

mean low 
water spring 

aduft larval mortality 
distribuí ron tacto rs 

adule larval 


Figure 8.1 The effeets of competition and predation on barnacle distribution in Scotland. 
C u intraspecific competition; C 2f interspecific competition between Chthamalus and Balanus, 
D, desiccation; P, predation by Nucella , a predatory snail. The widths of the distribution bars 
indícate relative abundance; the widths of the mortality bars indícate relative importance of 
the factors concerned. Note that the upper limits of distribution for both species are 
determined by physical factors (i.e. tolerance to dessication). Snail predation and 
intraspecific competition for space are the major causes of mortality for Balanus, and both 
factors become increasingly important at lower tidal levels. For Chthamalus, the major cause 
of mortality of newly settled larvae is intraspecific competition for space with the 
faster-growing Balanus . Few Chthamalus larvae settle below mean tide level, but those that 
do are eliminated by predation and interspecific competition. 

Predation too may be an important determinant of zonation pattems. The 
whelk Nucella lapillus is a major predator of bamacles in Scotland. Like 
many predators, this snail prefers to eat larger prey, and thus prefers Balanus 
balanoides over Chthamalus. When cages were used to exelude Nucella 
from natural populations of barnacles, it became evident that snail predation 
was a major cause of mortality for larger (and older) Balanus , especially 
those living in the lower intertidal zone where Nucella was most abundant. 
Thus the lower limit of the Balanus zone is determined largely by predation. 

Similar predator-prey relationships can be found on the west coast of North 
America between three barnacle species ( Chthamalus dalli , Balanus glándula 
and B. cariosas) that are preyed upon by three different species of Nucella. 
In this area, most of the mortality of young Balanus glándula is the result of 
predation rather than crowding and competition for space, and B. cariosus 
attains an adult size that is too great to be eaten by Nucella. However, here 
too predation and competition for space act to set the lower limits on the 
barnacle distributions. 

Physical and biological factors also act in concert or independently to set the 
zonation patterns of benthic algae. Algae compete for sunlight (see 
Section 7.1) and for restricted space with other plants and with animáis, and 
these biological factors partly establish the position of plants on shores. 
Upper zonation limits of algal species are often set by tolerances for 
exposure and desiccation, and may also be determined by the grazing 
pressure of herbivores. As an example, the Torrey Canyon oil spill in 1967 
killed the dominant grazing molluscs in the intertidal areas of parís of 
south-western England, and subsequently there was a rise in the upper 
zonation limits of several intertidal algae. With eventual recovery of the 
grazers, the higher reaches of the algae were once again grazed down and 
the original zonation pattern was re-established. 



Both benthic algae and phytoplankton are important primary producers 
supporting rocky intertidal communities, but production figures are relatively 
low. The intertidal zone is a difficult habitat for benthic algae in several 
respects. In tropical regions, heavy rainfall, high light intensities, and 
exposure to high air temperatures with resulting desiccation are major 
problems. Freezing and scouring by ice limit algal production in arctic and 
subarctic intertidal areas. In températe climates, where benthic algae reach 
their full potential, there is competition between algal species for access to 
sunlight, and competition for attachment sites with other algae and with 
sessile animáis. The average annual productivity of the rocky intertidal areas 
of the world is of the order of 100 g C m~ 2 . However, production rates of 
around 1000 g C m~ 2 yr^ 1 may occur in particularly favourable areas. 

Attached algae are grazed by a variety of molluscs and sea urchins. Mussels, 
barnacles, clams, tunicates, tubeworms (polychaetes), and sponges are 
among the many filter feeders that are dependent on plankton. Intertidal 
carnivores inelude starfish, which eat limpets, snails, barnacles, mussels, and 
oysters; predatory snails, which consume a variety of prey including clams, 
mussels, and barnacles; and sea anemones, whose prey ineludes shrimp, 
small fish, and worms. Important scavengers inelude isopods and crabs. 
Shorebirds may also have considerable predation impaets on intertidal life 
(see Sections 6.5 and 8.5). 

Experimental work has demonstrated that herbivores such as sea urchins, 
limpets, chitons, and littorine snails may control both the level of primary 
production and the species composition of benthic plants. For example, 
removal of limpets from experimental areas results in the appearance of 
different species of algae and in heavier algal growth compared with 
undisturbed sites. Removal of sea urchins from intertidal and subtidal 
regions also tends to create a greater initial diversity of algal species, 
although this may eventually change to lower diversity as other determinants 
of community structure come into play. The species composition of the algae 
in a community may also result from competition for space and light 
between different algal species, with the dominant species being those that 
are fastest growing in the particular locality. 

Competition and predation are also important determinants of species 
composition and diversity among intertidal animáis. Along the north-west 
Pacific coast of North America, the rocky intertidal community is dominated 
by mussels, barnacles, and the carnivorous starfish Pisaster ochraceus. 
Pisaster feeds on a variety of molluscs and barnacles, as illustrated in 
Figure 8.2. Experimental removal of the starfish from the community 
resulted in lowering species diversity from about 30 species to one dominant 
species, the mus sel Mytilus califo miarais. When Pisaster is present, its 
feeding activities control the numbers of dominant sessile prey (barnacles 
and mussels) so that space is kept open and none of these species becomes 
dominant; at the same time, primary production is also enhanced by the 
provisión of space for benthic algae. When the top predator is removed, 
competition for space is intensified and, in this región, Mytilus californianus 
overgrows and outeompetes all other macrobenthos to take over the available 
space throughout most of the mid-intertidal zone. Pisaster is referred to as a 
keystone species because its activities disproportionately affect the patterns 
of species occurrence, distribution and density in the communities in which 


Figure 8.2 The food of the starfish Pisaster 
ochraceus, a keystone species of rocky intertidal 
communities along the Pacific coast of North 
America. Numbers in parentheses indícate the 
number of species in a particular group. 
(Animáis are not to scale.) 

it lives. Similar types of interactions control rocky intertidal community 
structure in other areas of the world. In New England, for example, the 
mussel Mytilus edulis is the competitively dominant sessile species, whose 
numbers are usually kept in check by two species of starfish ( Asterias 
forbesi and A. vulgaris) and by the snail Nucella lapillus . 

QUESTIOM 8.1 Would you expect to find a greater biomass per unit area of 
benthic organisms in intertidal areas with a high tidal ranga {e.g. >2 m). or 
in those with a small tidal range (e.g, <0,5 m), and why? 


ln coid températe regions, intertidal rocky-shore communities merge 
subtidally into kelp forests. The term kelp refers to a variety of very large 
brown algae that are usually found only outside the limits of the 20°C 
summer isotherms. These algae form distinctive subtidal communities in 
areas of upwelling, fast currents, or heavy surf. Kelp require a hard substrate 
for attachment, and they grow off rocky shores to depths of 20-40 m, 
depending on water clarity. Kelp beds occur along the western coasts of 
North and South America, extending into subtropical latitudes in upwelling 
areas. In the western Pacific, extensive kelp beds are found off Japan, 
northem China, and Korea. In the Atlantic, large kelp beds occur off the 
Canadian east coast, and off the coasts of Southern Greenland, Iceland, and 
northem Europe including the United Kingdom. The highest biomass levels 
of kelp are around subantarctic islands like the Falklands. New Zealand and 
South Africa are also localities that support kelp in quantities sufficient for 

Each kelp plant typically has a holdfast for attachment to the substrate and a 
flexible stipe (or stalk) (Figure 8.3). Large, thin blades (equivalent to leaves) 
are attached to the stipe. Kelp may also have gas-filled floats (or 
pneumatocysts) which keep the blades near the water surface where solar 
radiation is highest. Because of their large photosynthetic surface and the 
constant supply of nutrients in the surrounding turbulent water, kelp are 
highly productive plants. Common Pacific genera inelude Nereocystis, 
Postelsia , and Macrocystis. Macrocystis pyrifera is commonly known as the 


Figure 8.3 A number of kelp species 
illustrating diversity of structure in this group of 
brown algae. (a) Nereocystis luetkeana, 

(b) Poste/sia palmaeformis, (c) Laminaria 

giant kelp as it may exceed 50 m in length, and it forms aquatic forests off 
California. Various species of Laminaria (usually 3-5 m in length) are the 
dominant kelp along Coastal areas of the North Atlantic, and they also occur 
as an understory species in the températe Pacific. 

Kelp not only are the largest of all algae, but some are considered to be 
among the world’s fastest-growing plants. Growth rates of 6-25 cm per day 
are common, and Macrocystis pyrifera grows as much as 50-60 cm per day 
off California. Kelp can be either annual or perennial species, some 
regrowing new stipes and blades from the original holdfast either yearly, or 
every few years. All kelp reproduce via production of spores. 

The high growth rates of kelp transíate into productivities of from about 600 
to more than 3000 g C rrT 2 yr -1 (compare with production valúes for rocky 
intertidal areas in Section 8.2.2). Off Amchitka Island in the Aleutians, 
annual kelp production is from 1300 to 2800 g C m -2 . This high production 
once supported populations of the giant Steller’s sea cow (see Section 6.4). 
Off Nova Scotia, in Atlantic Cañada, Laminaria forests produce about 
1750 g C m -2 yr -1 . Kelp beds off South Africa have a production of about 
600 g C m -2 annually. In some locations, kelp are harvested for fertilizer, 
iodine salts, industrial Chemicals, and alginates used as food additives. The 
commercial harvest of the California giant kelp amounts to 10000-20000 
tonnes dry weight per year. 

Kelp communities provide spatial heterogeneity and diverse habitats and thus 
support a highly diverse association of animáis. The large surface area of the 
kelp blades provides space for numerous epifiora and epifauna, including 
diatoms and other microflora, and colonial bryozoans and hydroids. A 
variety of molluscs, crustaceans, worms, and other animáis live on the 
plants, or on the substrate between plants. In some areas, much of the 
primary production may be consumed by herbivores such as sea urchins. 
Some snails and sea slugs (e.g. Aplysia) also feed on kelp directly, but 
usually consume only vejy small amounts of the total production. This 
habitat also supports a variety of fish that feed on kelp-associated animáis 
and that find protection from predators such as seáis, sea lions, and sharks. 

In many areas, however, as much as 90% of the kelp production is not 
consumed but enters the detritus food chain. The edges of the kelp are 
continually abraded by wave action, with small fragments being torn off, and 


there may be self-thinning with some plants losing the competition for 
available light, space, and nutrients. Annual kelp species (e.g. 

Nereocystis spp.) may attain a summertime biomass of more than 100 tonnes 
per hectare, all of which may be destroyed in the first winter storm. This 
material enters the detrital pool of the kelp bed or is exported to other areas. 
Kelp that is uprooted in storms may wash up in large quantities on beaches, 
where it is eaten by amphipods or isopods. Kelp also release considerable 
quantities of organic matter into solution, and this exúdate is utilized by 
bacteria and thereby converted into particulate biomass (see Section 5.2.1). 

The sea otter ( Enhydra lutris) (Colour Píate 32) is considered to be a 
keystone species in North Pacific kelp forests. Otters eat a wide variety of 
prey including crabs, sea urchins, abalone and other molluscs, and 
slow-moving fish, and a single otter may eat 9 kg of food per day. Off 
Amchitka Island, otters at a density of 20-30 km -2 annually consume about 
35 000 kg krrr 2 of prey. Otter predation on sea urchins regulates the 
ecological balance between kelp production and the destruction of the kelp 
by the herbivorous urchins. 

Sea urchins ( Stronglyocentrotus spp.) graze directly on living kelps, and they 
are capable of eating through the holdfasts that anchor the algae to the 
seafloor. The detached kelp is then swept away from the area in ocean 
currents. Otters, by direct predation, maintain relatively low densities of sea 
urchins and thus protect the kelp from overgrazing. The importance of otters 
in maintaining healthy kelp forests was revealed from ecological 
comparisons of different islands in the Aleutian chain. Certain islands off the 
Alaskan coast were found to have lush kelp communities with thriving 
populations of otters, seáis, and bald eagles; other nearby islands had no kelp 
or otters, and few seáis or bald eagles. The depaupérate islands were the 
focus of extensive hunting expeditions during the eighteenth and nineteenth 
centuries, and historical records document the elimination of otters from 
many kelp beds during this time. Only those islands which had been 
repopulated with the few surviving otters had flourishing kelp forests. Since 
1911, sea otters have been protected by law, and populations have recovered 
in some areas of Alaska and British Columbia. They have also been 
deliberately reintroduced into other depopulated regions, including along the 
California coast. Where otter populations have recovered, sea urchin numbers 
have decreased, and kelp production has increased. Despite protection, otters 
remain a vulnerable species. They are disliked by fishermen, who often 
perceive them as competitors for fish or shellfish (especially the valuable 
abalone). Otters are also particularly vulnerable to oil spills, as the Exxon 
Valdez experience of 1989 demonstrated when at least 5000 otters were 
killed by exposure to crude oil spilled off the Alaskan coast. 

Wherever sea urchins occur in very high densities in kelp beds, they are 
capable of eliminating the kelp as illustrated in Figure 8.4. Prior to 1968 
there were luxurious beds of Laminaria along rocky shores of the Nova 
Scotian coast (eastern Cañada). The kelp beds extended to a depth of about 
20 m, and they supported sea urchin populations of about 37 individuáis 
m -2 . From 1968 onward, the urchins (Stronglyocentrotus droebachiensis) 
became more and more abundant, and barren areas developed where the 
kelps were eliminated. By 1980, urchin-dominated barren grounds extended 
along more than 400 km of coastline. The rocky substrate became encrusted 
with coralline red algae, which are not controlled by urchin grazing. In the 
early 1980s, the urchin population was decimated by epidemic disease and 


Figure 8.4 The biological events that altérnate 
between productive kelp beds and barren 
grounds that are dominated by sea urchins. 

kelp began to reappear along most of the coast. Within three years of the 
mass mortalities of urchins, luxuriant kelp beds had been re-established in 
some areas. 

Anecdotal evidence from fishermen suggests that mass mortalities of sea 
urchins, and reciprocal fluctuations in kelp and urchin abundance, have 
occurred off the Nova Scotian coast since at least the turn of the century. 
The population explosions of sea urchins may be related to very high 
recruitment success of larvae in certain years, and this may be related to 
changes in local seawater temperature. As well, higher than average water 
temperatures have been linked with outbreaks of disease in urchins. These 
facts suggest that fluctuations in kelp and urchins may be natural events 
triggered by environmental change, and that they may have been occurring 
over a very long time. In any case, it is clear that urchin grazing and disease 
regúlate the ecological dynamics of these subtidal Laminaria communities. 


The intertidal zones of sandy beaches appear barren when compared with 
rocky shores. In particular, beaches exposed to severe wave action often 
seem entirely devoid of life. This is because the nature of the substrate sets 
living conditions which are best met by infaunal organisms that usually 
remain hidden from direct observation. This makes sand beaches more 
difficult to study than areas where the activities of the resident organisms can 
be directly observed. Compounding the problem is the fact that many of the 
organisms in this environment are of very small size, making their separation 
from the sediment tedious, and their taxonomic identification difficult. 


Beach sand grains are usually formed of irregularly-shaped quartz particles 
mixed with a high proportion of shell fragments, and with detritus derived 
from both marine and terrestrial sources. The particle size of sand varies 
from <0.1 mm to >2 mm and is determined largely by the degree of wave 
action; protected beaches have finer sand particles than exposed areas, where 
waves resuspend and transport small-sized grains. There is a gradient in 
particle size between sand and mud, with mud being composed of finer 


partióles and mudflats being formed in areas of little water movement (see 
Section 8.5). Some substrates are difficult to characterize, giving rise to 
terms like muddy sand, or sandy mud. As partióle size increases, sand grades 
into gravel or shingle. These large-particle substrates do not retain water 
because of their high porosity, and the shifting and abrasión of the large 
partióles also contribute to an absence or low diversity of life on gravel and 
shingle shores. 

Sandy beaches typically have a gradual slope, and this means that the 
sediment drains and dries out relatively slowly. Although oxygen is plentiful 
in the overlying water, oxygen contení in the substratum diminishes with 
depth because of the respiration of micro-organisms and the oxidation of 
Chemicals within the sand. Anaerobio conditions are marked by a black 
sulphide layer beginning at a depth of from a few millimetres to nearly a 
metre, depending on the organic content of the sand. Chemosynthetic 
bacteria are present in the sulphide layer (refer to Section 5.5). 

Animáis require special adaptations to live in an environment where the 
substrate is physically unstable in the sense that the sand grains are 
continually moved by turbulent water. The continual shifting of the surface 
layers of exposed beaches exeludes large sessile species and most large 
epifauna in general. As well, sand beaches contain relatively low 
concentrations of organic matter. On the other hand, sand buffers against 
large temperature and salinity fluctuations, and organisms burrowing into 
sand are kept moist at low tide. Sand also acts as a protective cover from 
intense solar radiation. Although there are differences in the physical 
environment and in the distribution of species from high to low tidal marks, 
the zonation patterns are not as clearly obvious as on rocky shores. Zonation 
on sandy beaches is also dynamic and variable; as the tide rises, many 
populations change their positions on the shore, or enter the water column. 


Primary producers 

There are no large attached plants below the high tide mark on sand beaches. 
The dominant benthic primary producers are diatoms, dinoflagellates, and 
blue-green algae. These are restricted to the near-surface layers of the 
sediment because light does not penétrate very deep in sand. The primary 
productivity of these benthic plants is very low (<15 g C m~ 2 yr -1 ), and the 
system depends primarily on energy derived from primary productivity in 
the surrounding water and on organic detritus. 


There is a low diversity of macrofauna compared with rocky shores or mud 
communities. Burrowing polychaetes and bi val ves plus crustaceans are 
usually the dominant members in terms of biomass. In températe latitudes, 
the supralittoral zone is occupied by air-breathing amphipods (beach hoppers 
or beach fleas) and, sometimes, also by isopods. Amphipods and isopods 
burrow into the sand during the day, and feed at night on detritus like 
decaying seaweed that has washed ashore. On tropical sand beaches, the 
highest reaches are occupied by ghost crabs ( Ocypode ) which are also 

The mid- and lower tidal zones support a higher diversity of macrofauna. 
Small, fast-burrowing wedge-shaped clams ( Donax , Tellina) are often present 
in vast numbers, some migrating up and down the beach with tidal changes. 


Larger razor clams (e.g. Ensis , Siliqua ) are also confined to sandy shores, 
and they too are rapid diggers. These mobile bivalves tend to have smooth, 
thin shells with slender profiles that ease passage through sand. Stouter 
thicker-shelled bivalves like cockles (e.g. Cardium) or Macoma also inhabit 
sand, but they tend to anchor themselves more firmly in the sediment and 
move less. The bivalves may be either suspensión feeders or deposit feeders, 
with some species capable of taking advantage of both food sources. In 
general, deposit feeders tend to dominate in fine-particle sands, presumably 
because the concentration of organic material is higher than in coarser sands. 
The sand environment is also home to certain snails that plough through the 
sand; these inelude the olive shells (e.g. Olivella ), and the larger moon snails 
(. Natica , Polinices ), all with smooth, undecorated shells. The majority of 
olive shells prey on small molluscs. Moon snails are also predators, 
especially of bivalves, and they gain access to their prey by drilling a hole 
through the shell. Where they are abundant, moon snails can have important 
effeets on community structure; when Polinices is experimentally removed, 
the numbers of clams and other infaunal prey increase. 

Few animáis form permanent burrows because wave action and the relatively 
large particle size of sand cause their collapse. However, exceptions can be 
found in those polychaete species that line their burrows with mucus or 
membranous materials. Many of the sand-dwelling polychaetes are deposit 
feeders, a few are suspensión feeders on plankton or resuspended organic 
material, and some (e.g. Nephthys, Glycera) are predators or scavengers that 
move through the sand actively seeking food. 

Characteristic crustacean inhabitants of mid-tidal levels inelude mole crabs 
of the genus Emérita . They typically lie with their entire body buried and 
only their antennae projecting above the surface of the sand to capture small 
suspended food particles from receding waves. Despite being swift 
burrowers, they are commonly preyed upon by shore birds. Prawns and 
mysids are other sandy-shore crustaceans; they burrow temporarily but 
emerge to feed. Predators of this community can inelude larger 
epifaunal crabs. 

Various types of echinoderms may be present at lower tidal levels, including 
burrowing sea cucumbers, heart urchins, and sand dollars, most of which are 
deposit feeders. The shortened spines of the sand dollars facilitate 
burrowing, and the young of some species ingest and selectively accumulate 
the heaviest sand particles containing iron oxide in their digestive tract. 

These increase the density of the small sand dollars and act as a weight belt, 
keeping them in the sediments when wave action is intense. Starfish are not 
common inhabitants in températe communities, but some tropical species 
(e.g. Oreaster) are present in low intertidal or subtidal depths where they 
feed on organic matter contained in the sand. 

Vertébrate members of sand beach communities inelude fish that are 
permanent residents, like sand eels that burrow into the sand at low tide, and 
temporary inhabitants (e.g. flatfish) that move into the area only at high tide 
to feed on smaller animáis. Shorebirds are also important predators of the 
community, and mammals (e.g. rats, otters) may visit this area to obtain food. 


The meiofauna of sand beaches ineludes some of the most diverse and 
highly adapted species in this environment. The term interstitial fauna is 
also applied to those animáis that live in the interstices, or spaces, between 


the sand grains. They either attach to sediment partióles, or move through 
the interstitial spaces without dislodging the grains. Many animal phyla are 
represented in this category, and some groups (e.g. gastrotrichs, see 
Figure 8.5c,d) are wholly or largely restricted to this particular environment. 
Biomass of the meiofauna usually ranges between 1 and 2 g m~ 2 , with the 
average number of individuáis being 10 6 m -2 (from the sand surface down 
to the anoxic layer). 

Figure 8.5 illustrates some of the characteristic meiofauna of sand. Many of 
their adaptations are morphological and can be seen in the figure; these 
inelude small size (only a few mm in largest dimensión even among groups 
that are usually large, such as echinoderms and molluscs), elongate or 
wormlike shapes, and flattened bodies. As well, many have a strengthened 
body wall as protection against crushing in a physically unstable substrate. 
This may involve the development of spines or scales (e.g. in gastrotrichs), a 
well-developed cuticle or exoskeleton (as in nematodes or crustaceans), or an 
internal skeleton of calcareous/spicules (some ciliates and sea slugs). 
Alternatively, soft-bodied animáis like ciliates, flatworms, and hydroids have 
developed the ability to contract strongly to protect against mechanical 
damage. Many of the interstitial species have special adhesive organs for 
maintaining a hold on the sediment particles; these may be epidermal glands, 
hooks, or claws (note Figure 8.5e in particular). 

The majority of the sand meiofauna are mobile, but some foraminiferans and 
hydroids (Figure 8.5h) remain firmly attached to sand particles. All feeding 
types are present, from animáis like ostracods and harpacticoid copepods that 
graze on benthic diatoms and dinoflagellates, to detritus feeders (gastrotrichs, 
nematodes), to predators such as hydroids and flatworms. Suspensión feeders 
are the rarest type, and these are sedentary animáis like bryozoans and 

Figure 8.5 Representative meiofauna from 
sand, all between 0.1 and 1.5 mm in length. 

(a) Psammodriíus (a polychaete); 

(b) Monobryozoon (a bryozoan attached to sand 
grains); (c) Dactylopedalia (a gastrotrich); 

(d) Urodasys (a gastrotrich); (e) Batillipes (a 
tardigrade); (f) Unela (a gastropod mollusc); 

(g) Pseudovermis (a gastropod mollusc); 

(h) Psammohydra (a hydroid attached to sand 
particles); and (i) Nerillidium (a polychaete) 


tunicates. The meiofauna fall prey to macrofaunal deposit feeders, shrimp, 
and young fish. 

Fecundity of the meiofauna is low owing to their small sizes and the 
consequent physical constraints on producing large numbers of gametes. 
Many of the species produce only one to ten eggs at a time, and about 98% 
of the species lack pelagic larvae. The young are often brooded by the parent 
until they are able to live freely or, alternatively, eggs are attached to the 
sand and the young hatch as benthic juveniles. Dispersal is by passive 
transport of those eggs or adults that are caught in water currents when the 
sand is washed away, or by organisms attached to sand partióles that adhere 
to the feet of wading birds. 

QUESIIQN 8.2 Aboui 98% of the meiofauna in sand do not produce 
planktonic larvae. What factors favour direct development and suppression 
of a pelagic phase in these species and in this environment? 


Estuaries are partially enclosed regions where large rivers enter the sea. They 
rank among the most productive of marine ecosystems as they typically 
contain a high biomass of benthic algae, seagrasses, and phytoplankton, and 
support large numbers of fish and birds. Estuaries are enriched by nutrients 
from land drainage, but their high productivity is also the result of nutrient 
retention within the estuary. This is due to the water circulation pattern that 
is set up when less dense freshwater overlies heavier salt water. Figure 3.15 
illustrates how estuaries tend to entrain nutrients from deep, saline water into 
the freshwater flowing seaward from the river, with the nutrient enrichment 
usually leading to a phytoplankton bloom seaward of the river mouth. Some 
of the bloom will sink out into the lower, more saline layers, and the 
decomposing phytodetritus will then be carried back toward the land. Thus 
the special circulation pattern of estuaries, combined with tidal flow, results 
in the sinking of particles and nutrients from seaward-flowing river water, 
and in these nutrients being carried back at depth in the saline water that 
fiows inward and upwells to replace that carried away by the surface flow. 

Each estuary has unique physical features that influence its ecology. These 
inelude the amount of river discharge, depth and general topography, specific 
circulation patterns, climatic regime, and vertical tidal range. Nevertheless, 
certain generalities emerge from the many comparative studies of life in 
estuaries. In several respeets, the estuarine ecosystem is much more complex 
than open ocean ecosystems, and the plankton community at the seaward 
edge of the estuary is only one of several communities governed by different 
groups of primary producers. The major components that typically make up 
estuaries are illustrated in Figures 8.6 and Colour Píate 33; the relative area 
occupied by each of these communities depends on local tidal action and the 
topography of the estuary. 

Starting from the upper reaches of temperate-latitude estuaries, there is firstly 
a sheltered, upper intertidal saltmarsh community dominated by a variety of 
marshgrasses (e.g. Spartina , Salicornia)\ this community is largely replaced 
by mangroves in tropical and subtropical latitudes (see Section 8.7). The 
marshgrasses, which are rooted flowering plants, may be as much as 2 m 
high, and they function as a trap for nutrient-rich sediment. Above-ground 







%m ^ 







or mangroves 

and seaweeds 



Birds and 

Snails and 







Birds, mammais, 





reptiles, etc. 




and fish 

and fish 



Harbour development, 

Disposal site for 



including dredging, 


sewage, dredged 

agriculture, etc. 

materials, etc. 

Figure 8.6 A schematic depiction of the communities composing the estuarine ecosystem, 
showing their dominant flora and fauna and potential human impacts. 

primary production of marshgrasses ranges from 200 to 3000 g C rrT 2 yr -1 , 
and production by benthic mud algae contributes another 100 to 
600 g C m -2 yr -1 . Thus saltmarshes rank among the most productive 
ecosystems on Earth. Most of the living plant material is not grazed directly, 
but enters detritus food webs either on the marsh or in adjacent waters. This 
plant debris decays slowly and, over long periods, the accumulation of debris 
and trapped sediment may create peat deposits that are several metres deep. 
This upward growth of saltmarshes results in changes in relative tidal level 
and drainage, and thus in changes in the species composition of plants; this 
process of marsh evolution eventually contributes to the infilling of estuaries. 

The upper reaches of a saltmarsh mark the transition between the sea and 
land. This habitat has great variations in salinity and temperature, and 
relatively few species of plants and animáis live here permanently. 

Terrestrial animáis such as raccoons, rats, and snakes invade this area, and 
there are large insect and bird populations. Faunal diversity is greater in the 
lower intertidal areas of the marsh, and the saltmarsh macrobenthos may 
inelude deposit-feeding fiddler crabs ( Uca) that build burrows in the mud, 
snails (e.g. Nassarius, Hydrobia, Littorina) that feed on the rich deposits of 
benthic diatoms, and mussels of the genus Modiolus that are specially 
adapted to live in or on mud and that can respire in both air and water. The 
leaves and stems of the marshgrasses serve as attachment sites for many 
small organisms, and significant numbers of micro- and meiobenthos live on 
or in the bottom sediments. Bacteria attain densities as high as 10 9 cm" 3 in 
the sediments, and they are an important food for protozoans and meiofauna. 
Saltmarshes fulfil the important function of providing shelter and food for 
shrimp, juvenile lobsters, and the young stages of many species of marine 
and estuarine fish. 

A seagrass community is located seaward of the saltmarsh, in the intertidal 
and subtidal zones. It may contain significant stands of seaweeds in addition 
to the seagrasses, but in general, seaweeds do not grow as well in muddy 
estuarine waters as they do in clear waters. The dominant plant of this 
estuarine community in températe latitudes is Zostera , commonly called 
eelgrass; in tropical climates, it is replaced by Thalassia , or turtlegrass, The 
brown seaweed Fucus and green seaweeds, Enteromorpha and Ulva , may 
grow on patches of rock among the seagrass beds. Measurements of the 


productivity of seagrasses are complicated by the fact that many epiphytic 
diatoms may grow on the blades of the seagrass, and these may add to the 
total primary productivity. For example, on the eastern coast of the United 
States, Zostera may produce about 350 g C m~ 2 yr" 1 , and associated plants 
contribute a further 300 g C m -2 yr -1 . Generally, the annual production of 
températe seagrasses is about 120-600 g C m -2 , while tropical seagrass 
communities have higher net primary productivities of up to about 
1000 g C m -2 yr -1 . 

Numerous meiofauna, including protozoans and nematodes, are associated 
with the seagrass epiphytes which are grazed by snails, isopods, amphipods, 
and harpacticoid copepods. Sessile filter-feeding invertebrates (e.g. hydroids, 
bryozoans, and tunicates) attach to the seagrass leaves. Snails, bivalves, 
polychaetes, and various types of crustaceans dominate the mobile 
invertebrate fauna of seagrass communities. This estuarine zone, like the 
saltmarsh, serves as a nursery area for the young of many species of fish, 
including commercial species such as menhaden and salmón. 

In both the saltmarsh and seagrass communities, little of the primary 
production is consumed by herbivores. Both communities are dominated by 
detritus-based food chains because marshgrasses and seagrasses contain large 
amounts of refractory material, such as cellulose, that is difficult for 
herbivores to digest. Less than 10% of the marshgrass is grazed by terrestrial 
herbivores, and usually only a small fraction of the seagrass production is 
eaten by such animáis as sea urchins and migrant birds (e.g. geese). 
However, in some tropical regions, turtlegrass may be consumed in large 
quantities by dugongs or manatees, and by sea turtles. In general, though, by 
far the largest fraction of the net primary production in both communities 
dies and is colonized by fungi and bacteria, to be converted eventually into 
microbial biomass. The numbers of bacteria in estuarine water are much 
higher than in seawater, and bacterial densities in sediments may reach 
200-500 x 10 6 per gramme of estuarine mud. Thus a large amount of plant 
detritus is produced, some of which is exported out of the estuary, and much 
organic carbón is recycled to re-enter the food chain through the microbial 
loop (see Section 5.2.1). Within the sediments, much of the organic matter is 
decomposed under anoxic conditions, with anaerobic bacteria using primarily 
inorganic sulphate as a source of oxygen. 

QUEST1QN 8.3 Is the occurrence of hydrogen sulphide in sediments an 
indicatíon of pollution? 

On the seaward side of the seagrasses, either a subtidal mudflat or sand-bar 
community will be present depending on the current and tidal regime. In 
fact, this community is continuous underneath both the intertidal seagrass 
and saltmarsh communities. The dominant primary producers of mudflats or 
sand bars are the epipsammic algae, which are generally species of benthic 
diatoms or dinoflagellates that are specially adapted to grow on sediment 
partióles. The surface of mud is sometimes colonized by thick mats of 
filamentous blue-green algae of several types. The productivity of this región 
tends to be inversely correlated with the grain size of the sediment particles, 
so that mudflats are generally more productive than sand bars in the same 
location. The primary productivity of these communities (in the absence of a 
cover of marshgrasses or seagrasses) tends to be low. Sand bars have a 
primary productivity of about 10 g C m -2 yr" 1 but mudflat production by 
benthic microphytes may be as high as 230 g C m -2 yr -1 . 

number of species 


QUESTI0N 8.4 Why should ihe partióle size of sand vs. mud affect the 
prodüctivity of the epipsammic algae? 

0 5 10 15 20 25 30 35 


Figure 87 An idealized diagram of the 
distribution of freshwater, brackish-water, and 
marine animáis relative to salinity. (Numbers of 
species given in relative units.) 

The mudflat community supports a wide range of animáis, with crabs and 
flatfish being common epifauna, and bivalves, polychaetes, and mud shrimp 
dominating the infauna. There is a rich meiofauna of small copepods, 
nematodes, and polychaetes, and an equally rich microfauna of protozoans, 
especially ciliates. Detritivores are usually predominant in the community. In 
the shallower reaches of this community, large numbers of birds eat the 
detritivorous invertebrates. Food consumption by birds may represent a 
significant impact on the mudflat species. For example, each of several 
thousands of knots ( Calidris canutus) on a large mudflat may eat as many as 
730 small clams ( Macoma) per day; a single redshank ( Tringa totanus ) may 
consume up to 40000 burrowing amphipods ( Corophium); and one 
oyster-catcher ( Haematopus ostralegus) can eat 315 cockles ( Cardium ) 
daily. Overall, birds may take between 4% and 10% of the accessible 
invertebrate fauna. 

The pelagic community located on the seaward edge of the estuary is 
controlled largely by the primary prodüctivity of the phytoplankton, and this 
ranges from about 100 to 500 g C m~ 2 yr -1 , depending on water clarity. 
Although nutrients may be plentiful, turbidity of the water often restricts 
light penetration and limits phytoplankton production. In shallow estuaries, 
as much as half of the phytoplankton may be consumed by filter-feeding 
benthos, with the rest being eaten by zooplankton. Zooplankton also may 
feed on benthic diatoms and bacteria-covered sediment partióles that are 
resuspended by intense mixing in shallow estuaries. In deep fjordlike 
estuaries, benthic plants are light-limited and most of the estuarine primary 
production is carried out by phytoplankton. 

Although estuaries are highly productive and host many juvenile fish, as well 
as large numbers of crustaceans, molluscs, shorebirds and waterbirds, the 
number of species found in these areas is relatively small compared with 
other marine habitats. Few species are adapted to cope with the salinity, 
temperature, and turbidity variations present in this habitat. Salinity tolerance 
plays a major role in the distribution of any particular species in an estuary, 
although distribution is also determined by such factors as substrate type and 
degree of tidal exposure. 

Figure 8.7 illustrates the typical distribution and relative diversity of 
freshwater, brackish, and marine animáis in relation to estuaries. Estuaries 
support an essentially marine fauna, but the number of marine species 
declines as the water becomes less saline, and the species change from those 
that are stenohaline to those that are euryhaline (see Section 2.3.2). The 
majority of animáis living in rivers do not tolérate salinities greater than 
about 0.5, and they do not penétrate further seaward than the uppermost 
reaches of the estuary. Only a few freshwater organisms (oligohaline 
species) can survive in water having a salinity of 0.5 to about 5; these 
inelude principally various insect larvae, oligochaete worms, snails, and 
some fish such as sticklebacks. There are relatively few brackish-water 
species that are restricted to estuarine conditions with salinities of about 
5-20, and most are animáis with marine affinities. Euryhaline marine 
organisms constitute the majority of species living in estuaries, and their 
distributions extend from the sea into the central regions of estuaries. 


Stenohaline marine species are unable to tolérate salinities lower than about 
25-30, and they are largely excluded from estuaries. Some fish (e.g. salmón, 
eels) are transient residents of estuaries, and move freely from the sea to 
rivers and lakes, or vice versa (see Section 6.6.1). Overall, estuaries have 
fewer species than adjacent aquatic environments, but abundance within 
individual species as well as biomass are often markedly increased. 

In general, the extent of penetration into estuaries by marine and, conversely, 
freshwater species is determined by the rate and magnitude of tidal change, 
rather than by the salinity gradient. That is, marine species penétrate farther 
upstream, and freshwater organisms reach much closer to the sea, in 
estuaries where tides are small and the salinity gradient is relatively stable. 
The minimum number of species occurs in that part of the estuary where the 
salinity variation is greatest. Finally, the distributions of benthic species 
within estuaries are also controlled by sediment type. 

QUESTION 8,5 Can you offer any explanation(s) as to why species diversity 
declines in estuaries relative to adjacent environments, but numbers of 
individuáis and biomass increase? 


Coral reefs are well known for their spectacular beauty (Colour Plates 34 
and 35), and they are perhaps the most diverse and ecologically complex of 
marine benthic communities. They are unique in being formed entirely by 
the biological activity of certain coráis belonging to the Phylum Cnidaria 
(see Table 7.1). These tropical reefs result from massive deposits of calcium 
carbonate laid down by the coráis o ver aeons of geologic time. These are 
among the oldest of marine communities, with a geological history 
stretching back for more than 500 million years. 


Living coral reefs cover about 600 thousand km 2 , or somewhat less than 
0.2% of the global ocean area and about 15% of the shallow sea areas within 
0-30 m depth. The largest reef is the Great Barrier Reef that extends along 
the east coast of Australia for a distance of more than 2000 km and is as 
much as 145 km wide. Reefs are located exclusively within water bounded 
by the 20°C isotherms and so are virtually confined to the tropics 
(Figure 3.10). Reef-building coráis cannot tolérate water temperatures of less 
than 18°C, and optimal growth usually occurs between 23° and 29°C, 
although some coráis tolérate temperatures of up to 40°C. A number of other 
physiological demands further limit the distribution of reef-building coráis. 
They require high salinity water ranging from 32 up to 42. High light levels 
are also necessary for reef-building (for reasons that will be explained 
below), and this restricts coráis to the euphotic zone. Even in the clear 
oligotrophic water of the tropics, most reef-building species live in water 
that is shallower than 25 m. The upward growth of a reef is restricted to the 
level of lowest tides, as exposure to air for more than several hours kills 
coráis. Coráis are also absent in turbid waters, as they are very sensitive to 
high levels of suspended and settling sediment which can smother them and 
clog their feeding mechanisms. High turbidity also affects reef-building by 
decreasing the depth of light penetration. New reefs are initially formed by 

the attachment of meroplanktonic coral larvae to a hard substrate, and for 
this reason reefs always develop in association with the edges of continents 
or islands. 

QUESTION 8.6 Refer to Figure 3JO. (a) Can you explain why coral reefs are 
generally absent on the west coasts of the Americas and Africa betvveen 30 S 
and 30 N? íb> Whai might preverá reef formación off nonh-easiern South 
America, northward froni the mouths of the Amazon and Orinoco rivers? 


Coráis are closely related to benthic sea anemones (both are in the Class 
Anthozoa) and are more distantly related to planktonic jellyfish, benthic 
marine hydroids, and the freshwater Hydra . Not all coráis are reef-builders; 
some are solitary or colonial animáis that are capable of living in deeper 
and/or colder water and are found throughout the world’s oceans. 
Reef-building stony coráis are colonial animáis, and each reef is formed of 
billions of tiny individuáis called polyps (Figure 8.8; Colour Píate 36). Each 
polyp secretes a calcium carbonate exoskeleton around itself that generally 
measures about 1-3 mm in diameter. Each polyp is equipped with tentacles 
containing batteries of nematocysts (see Section 4.2), and these stinging cells 
can be used to capture prey and for defence. The polyps can produce a large 
colony by asexual división, or budding, and all the polyps in a colony 
remain connected to each other by extensions of their tissues. Coráis also 
reproduce sexually, producing planktonic larvae that disperse, settle, and 
establish new colonies. 

Individual coral colonies vary in size, but some are very large, weighing up 
to several hundred tonnes. The form of a colony, whether it is branching, 
massive, lobed, or folded, depends on the species and also on the physical 


Figure 8.8 Anatomy of a coral polyp. The animal is basically a contractile sac housed in a 
carbonate skeleton. The central mouth is surrounded by six, or a múltiple of six, tentacles 
equipped with batteries of nematocysts. The tiny zooxanthellae live in cells in the lining of 
the central digestive cavity. Each polyp secretes a protective carbonate exoskeleton consisting 
of a radial arrangement of vertical plates; as it grows upward, the polyp deposits new layers 
under itself. 


environment in which the coral is located. The same species may have a very 
different form when it grows in areas exposed to wave action as opposed to 
calm conditions, or when it grows in shallow versus deeper waters. 


The diversity of Ufe on a coral reef is extraordinarily rich. Figure 8.9 
illustrates only a very few dominant types of the coral-reef fauna. The Great 
Barrier Reef is composed of about 350 species of hard coráis, and is home 
to more than 4000 species of molluscs, 1500 species of fish, and 240 species 
of seabirds. In addition, there are many more species of macrobenthos, and 
the numbers of micro- and meiofauna remain unknown. Representative 
species of almost all phyla and classes can be found in the reef ecosystem. 

Figure 8.9 A coral reef habitat illustrating some of the many inhabitants of this diverse 








nudibranch (sea slug) 






lobed coráis 


colonial tunicate 


sea whips (gorgonian coráis) 


giant clam ( Tridacna) 




pseudochromid fish 


sea fans (gorgonian coráis) 




tube anemone 


soft coráis 


stone coral 


cleaner shrimp 




sea anemones 


brain coral 




butterfly fish 


worm tubes 


moray eel 


snail (cowry) 


cleaner fish 


sea fan (gorgonian) 


tube coráis 


Reefs in the Indo-Pacific have a high diversity of coral species, with at least 
500 reef-building species throughout the entire región. Atlantic reefs are 
impoverished in comparison, with only about 75 species of reef-building 
coráis. The number of species in other animal groups associated with reefs is 
also generally lower in the Atlantic sector than in the Indo-Pacific. The 
number of mollusc species is estimated at about 5000 in the Pacific versus 
1200 in the Atlantic, and there are about 2000 versus 600 fish species in 
these respective reef areas. The differences in species diversity may result 
from differences in the age of the oceans, and the respective geologic times 
over which reefs have evolved. Geologically, the Atlantic is a more recent 
ocean, and its reefs were also more severely influenced by decreased 
temperatures and lowering sea levels during ice ages. Most Atlantic reefs are 
only 10000-15 000 years oíd, these dates corresponding to the last glacial 
age. In contrast, the Great Barrier Reef is about 2 million years oíd, and 
some Pacific atolls date back about 60 million years. 

The reef itself provides food and shelter for many plants, invertebrates, and 
fish. For sessile species, the reef offers a site of attachment. Surface 
irregularities in the reef limestone create a variety of microhabitats like 
crevices and tunnels, and these contribute to the faunal diversity of the 
system. Areas of rubble and sand also accumulate between coral heads, and 
these sediment types require different sets of adaptations and develop 
different communities from that associated with the hard-substrate reef. A 
reef is also differentiated into regions distinguished by physical differences 
in wave action, depth, and degree of tidal exposure. This wealth of different 
habitats is a major factor in supporting the many species of a reef. 

Coral polyps usually dominate the living biomass of a reef, but other reef 
organisms also contribute to the carbonate reef structure. These inelude the 
hard, coralline red algae that grow in thin layers over the surface of the reef. 
These encrusting algae precipítate CaCC >3 and play a role in cementing the 
reef fragments together. Some green algae also secrete calcium carbonate, 
other green algae do not. In addition to encrusting algae, there are benthic 
algae that are erect species, and some that live within the spaces of the coral 
framework. Seagrasses often grow in the sandy areas within or surrounding 
the reef. All of these plants provide food for herbivorous species of 
invertebrates and fish. However, the algae are generally inconspicuous 
inhabitants of the reef, and animal life is visually dominant. 

In addition to the reef-building stony coráis, other types of cnidarians are 
prominent reef members (see Figure 8.9 and Colour Píate 37). These inelude 
several types of non-reef-building coráis, including tire coráis, pipe coráis, 
and soft coráis. Sea whips and sea fans are also common reef inhabitants; 
they are cióse relatives of stony coráis and have internal skeletons composed 
of calcareous spicules. Other major in vertébrate groups in a reef community 
inelude echinoderms (starfish, sea urchins, and sea cucumbers), molluscs 
(limpets, snails, and clams), polychaete worms, sponges, and crustaceans 
(including spiny lobsters and small shrimp). Some of the invertebrates are 
encrusting species, like bryozoans; some build calcareous tubes, like certain 
polychaete worms; and some snails attach tube-like shells to the reef. All of 
these activities serve to cement the limestone reef framework together. In the 
Pacific, giant clams belonging to the genus Tridacna are also important 
structural components of reefs (Figure 8.9). These molluscs contribute an 
astonishing biomass to the reefs, as they grow to over 1 m in length and 
may exceed 300 kg in weight. 


Fish comprise the dominant vertebrates on a reef. Many of the reef fish are 
brightly coloured and visually conspicuous. About 25% of the world’s 
species of marine fish are found only in reef areas. These diverse species of 
fish show a high degree of feeding specialization and food selection. Some 
are herbivores, feeding on algae or seagrasses; some specialize in being 
plankton-feeders; and some are piscivorous, or are predators of benthic reef 
in vertebrates. Fish not only play important ecological roles in grazing or 
predation, but the faeces of these abundant animáis contribute an important 
source of nutrients to the reef ecosystem. 

The very large number of reef species, and the abundance of life on the reef, 
lead to intense competition between species and between individuáis for 
limited resources. The high degree of food specialization observed in many 
reef species is a reflection of the high species diversity of the reef, and every 
available food resource is efficiently utilized. There is also intense 
competition for space on the reef, and every microhabitat is occupied by 
organisms adapted to their particular site. Experimental work has revealed 
that the mesenterial filaments (Figure 8.8) of some coráis contain substances 
that kill polyps of adjacent colonies. Aggressive, slow-growing coráis can 
thus avoid being overgrown by less aggressive, but faster-growing species. 


Reef-building coráis are also called hermatypic coráis. They are 
distinguished from non-reef-building (ahermatypic) species by having a 
special symbiotic association with certain algae. Each hermatypic coral 
polyp contains masses of photosynthetic dinoflagellates, called 
zooxanthellae. These are a vegetative form of free-living dinoflagellates; 
when cultured under laboratory conditions, they develop into motile 
flagellate forms identical with planktonic dinoflagellates (see Section 3.1.2). 
The zooxanthellae in all coráis belong to a single genus, Symbiodinium , with 
different species or strains being specific to particular coral species. The 
zooxanthellae Uve within cells in the lining of the gut of coráis, reaching 
concentrations of up to 30000 cells per mm 3 of coral tissue. Under stressful 
environmental conditions, the symbiotic algae can be expelled from the 
coral. Because much of the colour of coráis is due to the pigmentation of the 
zooxanthellae, this expulsión is referred to as ‘bleaching’. 

The algal-coral relationship is beneficial to both species. The coral provides 
the algae with a protected environment, but it also provides certain Chemical 
compounds that are necessary for photosynthesis. Carbón dioxide is 
produced by coral respiration, and inorganic nutrients (ammonia, nitrates, 
and phosphates) are present in waste producís of the coral. In return, the 
algae produce oxygen and remo ve wastes; but most importantly, they supply 
the coral with organic producís of photosynthesis that are transferred from 
the algae to the host. These Chemical producís inelude glucose, glycerol, and 
amino acids, all compounds that are utilized by the coral polyps for 
metabolism or as building blocks in the manufacture of proteins, fats, and 
carbohydrates. The symbiotic algae also enhance the ability of the coral to 
synthesize CaCCT}. Rates of calcification are significantly slowed when 
zooxanthellae are experimentally removed from coráis, or when coráis are 
kept in shade or darkness. The relationship between the two independent 
processes of CO 2 fixation by photosynthesis and CO 2 fixation as CaCC >3 is 
complex and not fully understood. However, the symbiotic association with 
photosynthetic dinoflagellates explains why hermatypic coráis require clear. 


lighted water. This association also leads to intense competition for space 
within areas of sufñcient light to support the zooxanthellae. 

qUESTION 8.7 Can any coráis grow below the euphotic zone? 

The coral-zooxanthellae symbiosis is maintained over time and distance 
because the algae are already contained in coral larvae before they are 
released from the parent polyp. This relationship is not unique on the reef, 
however. Zooxanthellae are also present in other reef inhabitants, including 
the majority of other cnidarians, some tunicates, some shell-less snails, and 
in the giant clam Tridacna. 

The symbiotic arrangement between algae and coráis or other invertebrates 
results in nutrients being tightly recycled within coral reefs. This intemal 
nutrient cycling is of primary importance in maintaining the productivity of 
the reef in oligotrophic tropical water. 

Symbiotic algae do not supply all the nutritional requirements of their hosts. 
All the animáis harbouring zooxanthellae are mixotrophic and capable of 
meeting their additional nutritional needs in other ways. Coráis are true 
carnivores that capture zooplankton, employing their nematocysts to paralyse 
the prey. Many coral species can also feed on suspended particles by 
producing mucous nets or mucous filaments to entangle food that is then 
drawn to the mouth by rows of cilia. Ciliary-mucus feeding extends the size 
range of potential food Ítems to inelude even bacteria. Coráis may also 
directly absorb dissolved organic matter. 

The relative importance of zooxanthellae versus captured particulate food to 
the nutrition of any particular coral probably depends on the particular 
species, and it will be influenced by the specific Chemical that is produced 
and translocated from the symbiotic algae to the host. It should also be 
influenced by various environmental parameters such as depth, light 
intensity, abundance of zooplankton, etc. 


Primary production in the coral reef System is carried out by the benthic 
algae attached to or associated with the reef, by the suspended 
phytoplankton, and by the zooxanthellae living within the animáis of the 
reef. This ecological fractionation of primary producers makes accurate 
measurements of primary productivity extremely difficult because different 
techniques must be employed for each. With the exception of the 
phytoplankton, it is also difficult to assess the standing stock of primary 
producers. To do so requires determining the plant/animal proportions of 
coral polyps and the relative contributions of various types of benthic algae 
to total reef biomass. Until these have been determined, the size of the 
primary producer trophic level remains uncertain. 

Production studies of coral reefs suggest that gross primary productivity 
ranges from about 1500 to 5000 gCm" 2 yr -i , valúes that are much higher 
than those of open tropical oceans (see Section 3.5 and 3.6). In fact, they 
represent some of the highest rates of primary production of any natural 
ecosystem. However, many of the nutrients contributing to this production 
are recycled (i.e. the /-ratio <0.1, see Section 5.5.1). Symbiosis between 
primary producers and dominant animal species of the community, with 


nutrients prevented from being washed away, is a dominant controlling 
feature of the biological production, just as it is in the deep-water, 
sulphide-communities which will be described in Section 8.9. 

Net primary production on reefs is lower than might be expected because 
respiration of the primary producers is high, with gross production to 
respiration ratios ( P/R ) usually ranging from 1.0 to 2.5. In comparison, 
healthy phytoplankton ha ve a P/R ratio of about 10. In addition, the 
coral-reef food chain is much longer than in upwelling zones (see 
equation 5.2), so that respiration losses throughout the entire ecosystem are 
high. This results in lowering the production of top-level predators relative 
to the high gross primary productivity. 


During the voyage of the Beagle in the 1830s, Charles Darwin observed that 
there were three basic types of coral reefs, and he formulated an hypothesis 
of reef formation that linked these types. His ideas are summarized below 
and illustrated in Figure 8.10. 

Reef formation is initiated with the attachment of free-swimming coral 
larvae to the submerged edges of islands or continents. As the coral grows 
and expands, a fringing reef is formed as a band along the coast or around 
an island. This type of reef is predominant in the West Indies (Caribbean 
Sea). It is also the first stage in the process of forming atolls. 

If the fringing reef is attached to the edges of a volcanic island or other land 
mass that is slowly sinking, while the coral continúes to grow upward, a 
barrier reef will eventually form. Rarrier reefs are separated from the land 
mass by a lagoon of open deep water. The Great Barrier Reef of Australia is 
the best known of this type, but it is in fact an aggregation of many reefs. 

Atolls mark the last stage in this geological process. When a volcanic island 
subsides below sea level, the coral reef is left as a ring around a central 
lagoon. Continued coral growth maintains the circular reef, but calm 
conditions and henee increased sedimentation in the central lagoon prevent 
development of a reef in this area. Hundreds of coral atolls are found 
throughout the South Pacific Ocean, all of them located far from land but 
attached to underwater seamounts (volcanic elevations rising from the 
seafloor) which have subsided with age. 

Darwin’s ideas on atoll formation were not substantiated until the 1950s, 
when drilling programmes on coral atolls encountered volcanic rock 
hundreds of metres below the surface. His hypothesis has been further 
supported by the discovery of seamounts, submerged far below the sea 
surface, that still have attached remnants of shallow-water coráis. 

QUESTIDN 8.8 Excluding pollution influentes, would you expect to tínd a 
difFerence in tola i biological production between a barrier reef located 
ofíshore of a continent and a mid-oceanic atoll? Explain your answer. 

The rate at which a reef develops depends on a balance between the growth 
rates (budding) and calcification of the coral polyps and the rates of 
destruction of the limestone framework. Coráis always grow upward, toward 
light, as each polyp deposits new carbonate layers under itself (Figure 8.8). 
Growth of the coral skeleton is much faster in sunlight than in darkness (and 


(a) fringing reef 



(b) barrier reef 

Figure 8.10 The íormation of coral atolls 
according to Darwin’s theroy of subsidence. 

(c) atoll 



therefore also faster in shallower water) and, not surprisingly, the rate of 
growth can be decreased if photosynthesis of the zooxanthellae is reduced by 
sediment-laden water or Chemicals (see Section 8.6.4). Growth rates may 
also decline with age and increasing size of a colony. In general, coráis are 
regarded as slow-growing, with measured rates of growth usually varying 
from <1 to 10 cm yr -1 . 

However, growth rates of individual coral species do not necessarily describe 
the rate of growth of an entire reef system. This is partly because different 


species of coráis have different growth rates, but also because growth and 
expansión of the reef is regulated by many other factors such as predation, 
competition for space with other organisms, and light intensity, to ñame only 
a few. Further, the limestone framework is continually being destroyed by 
biological activities and physical events (see below). Estimates of total reef 
growth can be made from measured changes in reef topography over several 
years, or from geological information on the thickness of reef limestone 
deposits. These estimates of net vertical upward growth of reefs vary greatly, 
from only a few millimetres per year, to 30 cm per 11 years under 
favourable conditions. 

In order to obtain better estimates of the rate at which entire reef Systems 
grow, it is also necessary to know something about the factors that destroy 
the reef and the rate at which the limestone is broken down. Reefs are 
subject to physical erosión by wave action and currents, and tropical storms 
can cause extensive damage. Reefs are also subject to continual bioerosion, 
or breakdown of the calcium carbonate skeleton by reef inhabitants. Some 
organisms associated with the reef remove part of the coral skeleton by 
boring into the reef, using Chemical dissolution or mechanical abrasión; 
these inelude certain species of algae, clams, sponges, sea urchins, and 
polychaete worms. Some animáis (e.g. herbivorous limpets and snails, 
parrotfish) remove pieces of the reef skeleton inadvertently during grazing. 
Small coral fragments are consumed by deposit-feeders such as sea 
cucumbers, and thus become further reduced in size. These destructive 
activities eventually break down reef material to fine-grained carbonate sand. 
Much of the fine-grained detritus is flushed away from the reef by waves 
and currents, but some accumulates in pockets between coral heads. 


All reefs exhibit zonation patterns resulting from a combination of bottom 
topography and depth, and different degrees of wave action and exposure. 
The patterns differ according to locality and type of reef, with atolls having 
the most complex zonation. The major divisions are illustrated in 
Figure 8.11 and discussed below, but depending on locality, the zones may 
be subdivided into as many as a dozen. 

The reef flat (or back-reef) is located on the sheltered side of the reef, 
extending outward from the shore or coastline to the reef crest. This area is 
only a few centimetres to a few metres deep, and large parís may be exposed 
at low tide. The width of the reef flat varies from a few tens to a few 
thousands of metres. The substrate is formed of coral rock and loose sand. 
Beds of seagrasses often develop in the sandy regions, and both encrusting 
and filamentous benthic algae are common. Because it is so shallow, this 
area experiences the widest variations in temperature and salinity, but it is 
protected from the full forcé of breaking waves. The reduced water 
circulation, accumulation of sediments, and periods of tidal emersión 
combine to limit coral growth. Although living coráis may be scarce except 
near the seaward section, this area of many microhabitats supports a great 
number of species in the reef ecosystem, with molluscs, worms, and decapod 
crustaceans often dominating the visible macrofauna. 

The reef crest (or algal ridge) lies on the outer side of the reef, with its 
exposed seaward margin marked by the line of breaking waves. As the ñame 
implies, the reef crest is the highest point of the reef, and it is exposed at 


Figure 8.11 A generalized cross section of a typical Caribbean fringing reef, illustrating the 
major ecológica! zones. 

low tide. The width of this zone varíes from a few to a few tens of metres. 
la some localities, encrusting red coralline algae are dominant; in other 
reefs, brown algae predomínate in this zone. Living coráis are very scarce 
where wave action is severe; usually only one or two robust coral species 
dominate in this región. 

The outermost seaward slope (also called fore-reef) extends from the low 
tide mark into deep water. The upper part of this zone is broken by deep 
channels in the reef face, through which water surges and debris from the 
coral reef leaves. Large coráis dominate here, and there are many large fish. 
The máximum number of coral species tends to occur at 15-25 m, then 
declines fairly rapidly with increasing depth. At 20-30 m depth, there is 
little wave action and the light intensity is reduced to about 25% of that at 
the surface; here, coráis tend to be smaller branched forms. At 30-40 m, 
sediments accumulate on the gentle slope and coral becomes patchy in 
distribution. Sponges, sea whips, sea fans, and ahermatypic coráis become 
increasingly abundant and gradually replace hermatypic coráis in deeper and 
darker water. At 50 m, the slope steepens into deep water. The depth limit 
for reef-building coráis is about 50-60 m in the Pacific, and about 100 m in 
the Caribbean; the difference is probably related to differences in light 


Mangrove swamps, also called mangáis, are a common feature covering 
60-75% of tropical and subtropical coastlines. These forests of trees and 
shrubs that are rooted in soft sediments occur in the upper intertidal zone. 
They produce a marine system that is similar to a saltmarsh in having aerial 
storage of plant biomass and in harbouring both terrestrial and marine 
species. The euryhaline plants making up this specialized community are 
tolerant of a wide range of salinities and are found both in fully saline 
waters and well up into estuaries, but they are restricted to protected shores 
with little wave action. The distribution of mangroves overlaps with that of 


coral reefs, but extends farther into subtropical regions. In many areas, 
mangrove swamps border coastlines protected by barrier reefs. 


The term ‘mangrove’ refers to a variety of trees and shrubs belonging to 
some 12 genera and up to 60 species of flowering terrestrial plants 
(angiosperms). Dominant genera inelude Rhizophora , Avicennia , and 
Bruguiera. They have in common the foliowing features: 

(a) They are salt-tolerant and ecologically restricted to tidal swamps. 

(b) They have both aerial and shallow roots that intertwine and spread 
widely over the muddy substrate in an impenetrable tangle (Colour 

Píate 38). The substrate is oxygen-poor, and the aerial roots allow the plants 
to obtain oxygen directly from the atmosphere. Many of the mangrove 
species also have special prop roots extending down from the trunk or from 
branches to serve as extra support. 

(c) Mangroves have special physiological adaptations that prevent salt 
from entering their tissues, or that allow them to excrete excess salt. 

(d) Many mangrove plants are viviparous, producing seeds that germinate 
on the tree. Young plants drop from the tree into the water, and the floating 
plants are dispersed by water. The Ufe eyele of these long-lived plants is 
illustrated in Figure 8.12. 

Some Indo-Pacific mangrove forests may contain 30 or more species of 
mangroves. There are fewer in Atlantic areas; a total of 10 species is 
distributed throughout the New World, and mangrove swamps in Florida, for 
example, support only three species. 

Figure 8.12 The lite eyele of viviparous mangrove trees. 



The physical environment of mangáis is characterized by considerable 
fluctuation in salinity and temperature. This is also a región that is strongly 
influenced by tidal action. Water exchange transports nutrients into mangrove 
areas, and exports material out. Tidal flow also results in an inflow and 
outflow of animáis, such as fish and shrimp, into the tidal area. Animáis 
living high in the intertidal zone are subjected to the greatest environmental 
variation and to potential desiccation. Nevertheless, the plants and animáis 
are adapted to tidally-induced fluctuation, and the largest mangrove swamps 
are in areas with a large vertical tidal range. 

Mangroves are found in regions of little wave action, and the intertwining 
roots of the plants further reduce water velocities. This results in trapping of 
suspended sediments and organic material (particularly leaves) which settle 
on the bottom to form black mud. The sediments tend to be anoxic because 
of high bacterial activity and because of poor circulation within the 
fine-grained substrate. 

There is a Progressive change from marine to terrestrial conditions from the 
seaward side of a mangrove area to the landward edge. There is a 
corresponding zonation of different mangrove species, based at least partly 
on their respective salt tolerances. 

Ecologically, a mangrove community can be divided into (a) the 
above-water forest, (b) the intertidal swamp, and (c) the submerged subtidal 
habitat. These distinctive zones support unique combinations of species 
which are described below. 

The above-tide forest formed by the trunks and leaf canopy of the 
mangroves is an arboreal environment inhabited by terrestrial species. Birds, 
bats, lizards, tree snakes, snails, land crabs and mangrove crabs, spiders, and 
insects are all common residents, with insects being the most diverse and 
most abundant. The birds and bats are mostly insectivores or are piscivorous, 
feeding on small fish. The crabs are detritivores or omnivores and may feed 
on marine prey during low tide. In some areas, domestic animáis (cattle, 
goats, or camels) may graze on the mangrove leaves. A study of Florida 
mangroves showed that about 5% of the total leaf production was consumed 
by non-mammalian terrestrial grazers, the rest entering the aquatic system as 
debris and becoming available for marine detritivores, either fish or 

The intertidal swamp offers a variety of different substrates and different 
microhabitats to support a more diverse community of marine species. Some 
organisms attach to the mangrove roots, others reside in or on the mudflat or 
mudbanks. Barnacles and oysters are conspicuous epifauna on the roots, with 
the latter often the dominant contributor to community biomass. Certain 
species of isopods bore into the woody prop roots, and their activities may 
sever the roots, although the total impact to the mangrove swamp is usually 
limited. Periwinkles (snails) are found in abundance crawling over the roots 
in the upper intertidal zone. Some polychaete worms are also associated with 
the root system, with some tube-building species attaching to this hard 
surface. In this area, combined densities of snails, nematodes, and 
polychaetes commonly exceed 5000 m -2 . 

The intertidal mudflat is the home of numerous burrowing fiddler crabs 
( Uca ), and sea cucumbers commonly are present on the surface of the mud. 
Both of these groups feed on detritus. Red and green benthic algae are 


grazed by amphipods and some species of crabs. Pacific mangroves are 
frequented by large-eyed mud-skippers (genus Periophthalmus ), fish that 
burrow into the mud but spend much time out of water, using modified fins 
to crawl on the mud fíat or, in the case of one species, to climb the 
mangrove roots. Various species of shrimp and fish move in and out of this 
región with the tides. 

Leaf fall is a major source of nutrients and energy in the intertidal swamp, 
and many of the residents are detritivores. Some remove suspended detritus 
by filter feeding (e.g. oysters), others feed on organic material in the 
sediments by deposit feeding (e.g. burrowing polychaetes), and others like 
crabs, shrimp, and amphipods capture larger particles of debris with their 
claw-like appendages. Most animáis in the community probably consume 
detritus in addition to living plant and animal tissue. 

The subtidal zone also has sediments of fine-grained mud with a high 
organic content, and sand patches may be present as well. The subtidal 
mangrove roots support a rich epiflora and epifauna of algae, sponges, 
tunicates, anemones, hydroids, and bryozoans, and their crowded numbers 
indicate that competition for space is intense on this substrate. In some areas, 
turtle grass ( Thalassia ) may be the dominant benthic plant, and it serves to 
stabilize the mud bottom. Burrowing animáis (e.g. crabs, shrimp, worms) are 
common, and their burrows facilitate oxygen penetration into the mud and 
thus ameliorate anoxic conditions. Fish are most common in this zone, and 
many of them are plankton-feeders. The fish, as well as crabs, lobsters and 
shrimp, form the basis for local fisheries. 

The primary producers in this system inelude not only the mangroves 
themselves, but benthic algae, seagrasses, and phytoplankton. Few 
production studies have been conducted in mangroves because they are a 
particularly difficult environment in which to work. However, it is clear that 
mangrove swamps are rich in reeyeled nutrients. Although large quantities of 
detritus may be exported from a mangrove community, the roots also trap 
organic-rich detritus which is broken down and decomposed in the 
sediments; the reeyeled nutrients then become available to be taken up by 
the roots of the mangroves. Thus the mangrove system is not solely 
dependent on nutrients dissolved in the surrounding oligotrophic seawater. 
Mangroves are also located in regions of intense solar radiation, and the 
combination of high nutrients plus high light should lead to high gross 
primary production rates. Plant respiration is variable and is possibly related 
to the degree of salinity stress in particular localities. However, it is 
estimated that mangrove swamps contribute between 350 and 
500 g C m“ 2 yr -1 net production to Coastal waters. 

QUESTIQN 8.9 How do valúes of nel primary producüvily in mangrove 
swamps compare with those íor phytoplankton production in tropical 
nuiriem-deficieni oeeanie waters'? Refer to Table 5.1. 


Mangroves figure importantly in the livelihoods of peoples living within or 
adjacent to these habitats. The trees themselves have traditionally been used 
for firewood and charcoal. The timber is water-resistant, so it is also used to 
construct boats and houses. The leaves are used for roof thatching and as 
fodder for cattle and goats. Even cigarette wrappers are manufactured from 
the young, unfolded, leaf sheaths of a certain mangrove species. 


Most of these tropical Coastal communities have long-standing fisheries 
based not only on fish like mullet, but also on the abundant populations of 
shrimp, crabs, bivalves, and snails. Fish nets and traps are often constructed, 
at least in part, from parts of the mangrove trees, and tannin extracted from 
the mangroves is used to increase the durability of fishing nets and sails. 

Mangroves also have great importance in non-commercial aspects. They 
form protective barriers against wind damage and erosión in regions that are 
subject to severe tropical storms. In some areas, mangroves may facilitate 
the conversión of intertidal regions into semi-terrestrial habitats by trapping 
and accumulating sediment. For example, mangroves have spread seawards 
at rates of between 100 m and 200 m per year in Indonesia. The root system 
also serves as a protective nursery ground for many species of fish, shrimps, 
juvenile spiny lobsters, and crabs. The forest canopy not only supplies food 
for many of the arboreal and marine inhabitants, either directly or as detritus, 
but it is utilized for nesting sites for a variety of tropical birds. 


The vast majority of the seafloor lies permanently submerged below tidal 
levels yet, relative to the intertidal regions, comparatively little is known 
about life in the bathyal, abyssal, and hadal zones (see Figure 1.1). This, of 
course, is due to their relative inaccessibility. Although it is possible to dive 
to several thousand metres in submersibles or to employ remote-controlled 
cameras, the number of hours of direct observations in the deep sea has so 
far been extremely low. Most information on deep-sea ecology comes from 
indirect inferences based on animáis contained in benthic samples obtained 
from ships. Whatever the method, expense is a limiting factor in deep-sea 
research. Few countries or institutions have submersibles to use for basic 
research, and few have large research ships equipped to obtain deep-sea 
samples. Collecting a sample from 8000 m depth with towed gear, for 
example, requires a very large winch with at least 11 km of cable in order to 
allow for the towing angle. It may take up to 24 hours to let out that much 
wire, obtain a sample, and then retrieve it. With large ship costs easily 
exceeding tens of thousands of dollars per day, a single sample containing a 
few benthic animáis can be beyond the budget of most oceanographic 
research facilities. Compounding this problem is the fact that animal life is 
just not very abundant in many deep-sea areas, so that it is desirable to have 
large numbers of samples. Nonetheless, new techniques for collection and 
observation, combined with accumulating numbers of analysed deep-sea 
samples, permit a general assessment of benthic life in deeper water. 

The deep-sea environment has been generally regarded as stable and 
homogeneous in terms of many physical and Chemical parameters. Water 
temperatures are generally low (from —I o to 4°C) and salinity remains at 
slightly less than 35. Oxygen content is also constant and is rarely limiting, 
with the exception of areas beneath upwelling zones or in some basins where 
water exchange is slight (e.g. the Cariaco Trench in the Southern Caribbean 
Sea). Soft bottom sediments, originating from land and/or from the sinking 
of dead planktonic organisms, cover most of the deep seafloor. Hard 
substrates are largely limited to mid-ocean ridges and seamounts that jut up 
from the sea bottom. Relative to surface currents, bottom currents in the 
deep ocean basins are slow (generally <5 cm per second) but more variable 
than once believed. Some areas experience abyssal (or benthic) storms 


lasting up to a few weeks, during which bottom currents increase in speed 
and may reverse direction. Deep boundary currents that move along 
continental margins may have velocities of up to 25 cm s _1 , and these may 
cause sediment resuspension and thus influence sediment redistribution. 
Deep-sea environments may experience seasonal variability in the amount of 
organic material that sinks from the euphotic zone to the seafloor. 


Most animal phyla are represented in this dark environment of low 
temperatures, high pressures, and predominantly soft substrates. It has been 
known since the time of the Challenger expedition that there are, however, 
changes in the relative abundance of different types of zoobenthos with 
increasing depth. Figure 8.13 is based on benthic samples taken from the 
Kurile-Kamchatka Trench in the North Pacific in the 1950s. It shows, for 
example, that sponges form a dominant component of the macrobenthic 
biomass between 1000 m and 2000 m, but they are small and scarce below 
2500 m. Starfish are important members of the trench community down to 
7000 m, at which depth they disappear. Holothurians (sea cucumbers), 
however, increase in relative abundance in deeper areas; one species (Elpidia 
longicirrata ) makes up about 80% of the total biomass in the trench. 

Globally, holothurians frequently domínate the biomass in depths over 
4000 m where sediments are relatively rich in organic matter. Numerically, 

Figure 8.13 The percentages oí different 
animal groups in the biomass of macrobenthos 
at different depths in the Kurile-Kamchatka 










^ 6000 









asteroids (starfish) 

■ sea anemones 



(C n ida ria-Ant hoza) 



echiurids and 





biomass (per cent) 


small burrowing polychaetes commonly make up 50-75% of the macrofauna 
in many, widely scattered, soft-bottom deep-sea sites. Small crustaceans 
(amphipods, isopods, tanaids) are also common deep-sea macrobenthic 
species, followed by molluscs (especially clams), and a variety of worms 
(sipunculids, pogonophora, echiurids, and enteropneusts). Brittle stars 
(ophiuroids) can be abundant in some areas; for example, they make up over 
60% by numbers of the macrobenthos in the Rockall Trough west of Ireland. 

Certain groups of animáis attain their greatest abundance and diversity in the 
deep sea. The soft-bodied or calcareous sponges that are common in shallow 
water are largely replaced by glass sponges with siliceous spicules in deep * 
water. Cnidaria are principally represented in the deep sea by sea anemones 
that live in burrows, and by sea pens and gorgonian coráis that may form 
densely populated beds under eutrophic waters where there is sufficient 
suspended material for feeding. Slender, branching colonies of black coráis 
have been found in the greatest depths. The Pogonophora are mostly a 
deep-water group found down to 10000 m, and echiurid worms become 
more common in depths exceeding 5000 m. Some echiurids reach body 
lengths of 1 m, and they can occur in dense aggregations in organically-rich 
sediments where they form a large proportion of the biomass. The more 
primitive crinoids, the stalked sea-lilies, are mostly restricted to deep-sea 

Benthic foraminiferans and related protozoans, the giant xenophyophores 
(see Section 7.2.1), increase in importance in deep water, both in terms of 
abundance and biomass. Unlike the shallow-living species with calcareous 
tests, deep-water foraminiferans have proteinaceous tests or exoskeletons 
made of agglutinated sediment particles. In certain areas, 30-50% of the 
seafloor may be covered by foram pseudopodia and, in the Aleutian Trench, 
forams comprise 41% of the meiofauna. Xenophyophores are known to 
occur in nearly all areas of the deep-sea basins at depths below 1 km. They 
may occur in densities of up to 20 m -2 , and they constitute up to 97% of the 
total benthic biomass in some areas of the South Pacific. 

Some animal groups show a tendency toward gigantism in the deep sea (see 
also Section 4.4); these inelude the benthic foraminifera and the 
xenophyophores, as well as certain amphipod species that attain lengths of 
about 28 cm. However, there is a reverse tendency in some groups toward 
miniaturization, and the deep-sea meiofauna is numerically dominant over 
the macrobenthos. Nematodes are ubiquitous in marine soft substrates and 
make up 85-96% of the deep-sea meiofauna. Harpacticoid copepods and 
ostracods are also common deep-sea members of this size category, the 
former group constituting 2-3% of the meiofauna in abyssal zones. Tanaids 
are extremely diverse in the deep sea, and many of the species are 
meiofaunal; in the north-west Atlantic, they occur in densities of 
about 500 m -2 . 

Certain animal groups are poorly represented in deep water. Decapod 
crustaceans (e.g. crabs, shrimp, lobsters), sea anemones, and echinoid 
echinoderms are absent or uncommon below about 6000 m. Fish are also 
rare in very deep waters; one of the deepest captured fish carne from 7230 m 
in the Kurile-Kamchatka Trench. These generalizations are largely based on 
collections made with dredges or trawls, both of which are difficult to use 
over rocky substrates or in relatively steep-walled trenches, and both of 
which can be avoided by swimming animáis. It is well to keep in mind that 
Jacques Piccard and Lieutenant Don Walsh, who together made the deepest 


dive in a bathyscaphe, observed flatfish and shrimp at over 10000 m; neither 
of these groups of animáis have been collected by conventional gear from 
such great depths. 

CUESTION 8.10 Whai explanatíons can yon give for why large sponges are so 
sueeesst'u) in shallower areas, and why sea cucumbers o fien domínate the 
macrobenthos of deep water? 

Some deep-sea residents have a cosmopolitan distribution and are found in 
all the major oceans; other species are restricted to relatively small areas. In 
general, species become more limited in geographic range as water depth 
increases. Only about 20% of the species present below 2000 m in the 
Atlantic Ocean are also found in the Pacific or Indian oceans. 

Table 8.1 Percentages of species living below 6000 m depth that are endemic to 
the hadal región. 



Number of 
hadal species 

% endemic 













Echiurid worms 



Sipunculid worms 
















































Many species found in areas deeper than 6000 m are endemic to the hadal 
región, and many are restricted to a particular trench. Table 8.1 lists the 
number of hadal species known in particular invertebrate groups, and the 
percentages of these that are endemic to this deep-sea región. Endemic 
species constitute up to 75% of the benthos in certain Pacific trenches. The 
high degree of endemicity suggests that trenches are fundamentally isolated 
habitats which are centres for the generation of new species. 

Trenches often have relatively high abundances of aplacophorans (wormlike, 
shell-less molluscs), enteropneust worms, and echiurid worms, all of which 
are poorly represented elsewhere. In the Aleudan Trench at depths of 
7000-7500 m, the macrofauna is dominated by polychaetes (49%), bivalves 
(12%), aplacophorans (11%), enteropneusts (8%), and echiurid worms (3%). 


The meiofauna is dominated by benthic foraminifera (41%), followed by 
nematodes (36%) and harpacticoid copepods (15%). Some of the zoobenthos 
of trenches exhibit palé coloration and are blind, characteristics that are 
shared with cave fauna. Very large size is also a characteristic of some hadal 
isopods, tanaids, and mysids. 

Deposit-feeding infaunal animáis are dominant in the soft, organically-rich 
sediments of the deep sea, usually comprising 80% or more of the fauna by 
numbers. At one site in the Atlantic, at a depth of 2900 m, 60% of the 
polychaetes, >90% of the tanaids, 90% of the isopods, >50% of the 
amphipods, and 45% of the bivalves are deposit feeders. Other groups, like 
sea cucumbers and sipunculids, also ingest the detritus and small organisms 
contained in surface or subsurface sediments. Because bottom currents are 
usually slow and do not disturb compacted sediments, the topographic 
features produced by these animáis persist for long periods. Faecal mounds, 
burrows, trails, and tubes are some of the biological features that are 
commonly recorded in the deep sea by remóte cameras. 

Animáis that feed on suspended partióles are also found in the deep sea, but 
they are much less abundant and are usually restricted to particular localities. 
This is because most of these epifaunal animáis require relatively firm 
substrates for attachment as well as high concentrations of suspended food 
partióles. As a result, many types of epifaunal suspensión feeders (e.g. 
common sponges, sea anemones, barnacles, mussels) show a marked 
decrease in abundance with increasing depth and distance from shore. They 
do flourish, however, on the rock substrates found on mid-ocean ridges and 
seamounts, and some may dominate in deep-sea sulphide communities (see 
Section 8.9). Relative proportions of deposit feeders and suspensión feeders 
vary throughout the deep sea according to the degree of organic enrichment 
of the sediments and the supply of suspended food. 

It is useful, in discussing deep-sea distribution of epifauna, to distinguish 
between two types of suspensión feeding. Active suspensión feeders (e.g. 
shallow species of sponges and tunicates) use their own energy to pump 
water through a filtering structure. These animáis are successful in 
environments where suspended particle concentrations are high enough to 
repay the energetic costs of pumping. Passive suspensión feeders (e.g. 
crinoids, some polychaetes, and most benthic Cnidaria like sea anemones 
and sea-fans) rely on external water currents to convey food to feeding 
appendages that are held into the flow. The passive feeders succeed only in 
environments where flow conditions are predictable and fast enough to 
supply them with sufficient particulate food. Suspended particle 
concentrations decrease with depth, but flow conditions become more 
predictable. Active suspensión feeders disappear as suspended loads 
diminish, and the large filter feeders in the deep sea are passive feeders. 
Depending on the current speed, the friction of water moving over the 
seafloor may create physical mixing of the bottom water; this benthic 
boundary layer extends from 10 to several 100 metres above the bottom. 
Turbulence in this layer can result in resuspension of bottom sediments; 
heavy inorganic partióles remain cióse to the seafloor, but suspended light 
organic partióles will reach máximum concentrations some distance above 
the bottom. Typical deep-sea passive suspensión feeders, such as sea-lilies 
(stalked crinoids) and bryozoan colonies, are found in highest densities 
where there is modérate current flow and resuspension of sediments. In 
contrast to their shallow-dwelling relatives, these animáis are often supported 


by long stalks which hold them well above the seafloor where concentrations 
of suspended organic material may be optimal. 

Some deep-sea groups have very different feeding mechanisms from those 
used by shallow-water related species. Glass sponges, for example, have an 
extremely porous body wall and water currents can enter passively as well as 
by active pumping. Members of one family (Cladorhizidae) of small-sized 
sponges occur in deep water to about 9000 m depth; at least some of them 
are highly modified carnivores that passively capture small swimming prey 
by means of filaments provided with hook-shaped spicules. Whereas 
shallow-water tunicates are active suspensión feeders and many are colonial 
species, deep-sea representatives tend to be solitary forms that may 
supplement active pumping with mucous nets held into currents to capture 
food. Some deep-sea tunicates are even more highly modified and have 
become carnivorous. 

In addition to deposit feeders and suspensión feeders, the deep-sea food 
chain ineludes many scavengers. Cameras have recorded the speed with 
which a variety of swimming animáis approach bait placed on the seafloor in 
deep water. These inelude giant amphipods, isopods, fish, and shrimp. Brittle 
stars and some polychaetes are among the slower-moving scavengers. Strict 
predators appear to be rare in very deep waters. However, diets of deep-sea 
benthic animáis are not well known; feeding type is usually inferred from 
anatomical structure and gut contents. 


The number of species of many types of macrobenthos (e.g. snails, clams, 
polychaete worms) and fish tends to increase with depth from about 200 m 
to 2000 or 2500 m, then declines rapidly with further depth. Based on these 
observations, it was believed for many years that deep-sea species diversity 
was low compared with that of shallow-water communities. However, the 
development and use of a new collection device, called an epibenthic sled, 
changed this perception. The epibenthic sled (Figure 8.14) was designed to 

Figure 8.14 An epibenthic sled designed to collect animáis living on or just above the 
seafloor. The mesh-size is small enough to retain meiofauna, and the sampler can be closed 
during retrieval so that the entire sample is retained. 


collect and retain smaller animáis, in a size category that was previously not 
well sampled. When this gear was first employed in the 1960s, a single 
collection sometimes contained more animáis than were collected by all the 
combined expeditions of the previous 100 years. To further demónstrate the 
effectiveness of this apparatus, one paper reported that over 120 new species 
of cumaceans (small crustaceans) had been collected. It soon became evident 
that the diversity of many smaller organisms increases with depth. For 
example, the number of species of meiobenthic copepods increases to at 
least 3000 m, and máximum diversity of benthic foraminiferans is found in 
depths exceeding 4000 m. 

It is now established that there is high species diversity in the deep sea, 
especially among the small infaunal deposit feeders. As additional samples 
are obtained from the deep sea and more new species are described, the 
more diversity in this area seems to approach that of highly diverse 
terrestrial environments, such as the tropical rain forest. Some researchers 
estimate that there may be more than one million species of marine benthic 
animáis, most of them living in deep-sea sediments. Species diversity does, 
however, vary in different oceanic areas. For example, in the North Atlantic, 
species diversity declines from the tropics toward north polar regions; but in 
the Southern Hemisphere, zoobenthos species diversity in the Weddell Sea 
(Atlantic sector of the Antarctic) is of the level normally associated with 
tropical regions. Deep-sea diversity also may vary according to different 
levels of surface primary production. In some areas, zoobenthos diversity is 
depressed under areas of upwelling and high surface productivity, probably 
as the result of reduced oxygen concentrations from decomposition of large 
amounts of organic material. 

As more areas of the deep sea are surveyed with increasingly sophisticated 
gear, it is becoming apparent that the environment itself, in terms of 
substrate features and/or current regime, is more diverse than was once 
thought. Environmental diversity in the form of microhabitats (small areas 
having slightly different environmental characteristics) can itself lead to 
higher diversity in animáis. Indeed, the deep-sea benthos is patchily 
distributed, with significant aggregations of animáis having been detected in 
different taxonomic groups on scales ranging from centimetres to metres to 
kilometres. This patchy distribution underscores the importance of obtaining 
representative samples when assessing biomass and species diversity of 
deep-sea animáis. 

8.8.3 BIOMASS 

Although the number of species is high in the deep sea, communities 
occupying the typical soft-sediment seafloor are characterized by low 
population densities and low biomass. Numbers of benthic individuáis (both 
macrofauna and meiofauna) per unit area tend to decrease roughly 
exponentially with increasing depth and, to a lesser degree, with distance 
from shore. Under the central oceanic gyres, the total density of macrofauna 
ranges from 30 to 200 individuáis per m 2 . With a few exceptions, the 
dominant infaunal species tend to be small as well as sparse. In the central 
North Pacific, meiofauna and microfauna dominate the benthos in numbers 
(0.3% and 99.7%, respectively), and in biomass (63.8% and 34.9%, 
respectively). Deep-sea biomass valúes do not inelude the larger demersal 
species, which are more difficult to capture and to quantify on an areal basis; 
their inclusión would undoubtedly increase the biomass valúes given here. 


Table 8.2 Average biomass valúes of benthic animáis at different depths. 

Depth range (m) 

Mean biomass 
(g wet weight m -2 ) 


3 x 10 3 

to 200 


















Average biomass valúes for different depth zones are given in Table 8.2. 
Benthic biomass is highest in shallow Coastal areas within the euphotic zone, 
and it is lowest under oligotrophic, central, oceanic regions. Keeping in mind 
that the average depth of the world ocean is 3800 m, most of the seafloor 
supports less than 5.0 g wet weight m -2 of living organisms. 

However, benthic biomass at any particular depth varíes according to the 
amount of organic material delivered to the seabed. This is reflected, for 
example, in the different levels of biomass found in various trenches, all of 
which are deeper than 6000 m. Trenches are located in regions of frequent 
seismic activity, and thus are subject to brief episodes of high sedimentation 
caused by slumping of sediments down the trench walls. This results in the 
deposition of organically rich sediments from shallower depths, but also in 
the burial of benthic communities. The biomass of hadal fauna may be very 
high in trenches that lie near large land masses as they receive land-derived 
sediments and organic matter, as well as organic material sinking from the 
overlying nutrient-enriched and highly productive surface water. Under these 
conditions, benthic biomass may range from 2 to 9 g wet weight m -2 at 
depths of 6000-7000 m in the Kurile-Kamchatka Trench (North Pacific) and 
in the South Sandwich Trench (South Pacific). Trenches far from land 
masses and under oligotrophic water (e.g. Mariana Trench) have very low 
biomass valúes of about 0.008 g m~ 2 . 

Benthic productivity cannot be assessed directly from biomass valúes, but 
many deep-sea species grow relatively slowly and their small biomass must 
indicate low productivity. Various estimates suggest that annual secondary 
production over most of the deep ocean floor is between 0.005 and 
0.05 g C m -2 . 

OUESTIÜN 8.1! How does the decline in benthic biomass vviih depth compare 
wiih the vertical distribotíon of zooplanklon biomass? Refer to Sectioti 4.4 
and Figure 4.14, 


Except for localized chemosynthetic production around deep-sea hot springs 
(see Section 8.9), there is no primary production in the dark, deep areas of 
the sea. Food availability, not low temperature ñor high pressure, limits 
benthic biomass in the deep sea. The deep-sea food chain is dependent on 
surface production, and only a small percentage (1-5%) of the food 
produced in the euphotic zone is transferred to the abyssal seafloor. The 


percentage diminishes with increasing depth because of the increasing 
probability that organic partióles sinking from the euphotic zone will be 
consumed or will decay before reaching the bottom. 

A variety of potential food sources sink from the productive surface zone 
and thus may become available to deep-sea benthos (Figure 8.15). The 
relative contribution of each of these sources depends on their sinking rates 
and attrition in intermedíate depths. 

1. Dead phytoplankton, zooplankton, fish, and mammals. In many areas 
of the ocean, much of the phytoplankton is consumed by herbivorous 
zooplankton in the euphotic zone. That fraction which is not eaten sinks very 
slowly because of its small size and, in very deep areas, it is lost through 
predation, disintegration, or decomposition at intermedíate water depths. In 
some regions like the North Atlantic, however, the phasing between seasonal 
phytoplankton blooms and zooplankton growth is such that much of the 
phytoplankton dies without being consumed (see Section 3.6), and the 
sinking phytodetritus may reach the seabed at depths down to 4000 m. The 
fate of most uneaten epipelagic zooplankton is similar, although sinking rates 
are somewhat faster because of their larger size. The corpses of large fish, 
squid, or marine mammals sink rapidly enough so that they may reach the 
seafloor in deep waters, where they become available to benthic scavengers; 
however, some are no doubt consumed by large animáis at intermedíate 
depths. In any event, the arrival of large animal carcasses on the sea bottom 
is generally an unpredictable and rare event, except perhaps under the 


seasonal migration routes of some fish and mammals. This is not the case, 
however, in heavily fished regions where tonnes of unwanted, incidentally 
captured fish (‘trash’ fish) are dumped back into the sea. The dumping of 
unmarketable by-catch may amount to a very significant fraction of the total 
reported fish catch, but it will only locally increase benthic food supplies. 

2. Faecal pellets and crustacean moults. Compact faecal pellets of some 
zooplankton (about 100-300 fim in size) are collected in mid-water traps 
designed to capture sedimenting particles, and their settling rates are such 
that they may reach the seafloor relatively intact. Faecal material of fish may 
also reach the seabed almost undegraded. Although some animáis ingest 
faecal pellets, these wastes generally contain large fractions of indigestible 
materials. Moulted exoskeletons of planktonic crustaceans occur in the 
benthic boundary layer, but they may result from deep-water species. Moults 
are also low in nutritional valué as they are composed primarily of chitin 
which cannot be digested directly by most animáis, but it is broken down by 
chitinoclastic bacteria living in the guts of many species. Faecal pellets and 
moults are colonized by bacteria during their descent in the water column, 
and they are eventually converted to bacterial biomass. Bacteria are 
important nutritional intermediates in the food cycle of the deep sea, and 
they comprise a major food source for benthic deposit feeders. There is, in 
fact, an increase in bacteria in the bottom sediments of the deep sea, with 
numbers exceeding one million per gramme of sediment between depths of 
4000 and 10000 m. 

3. Macrophyte detritus. A certain amount of organic material enters the 
sea near Coastal zones in the form of wood from terrestrial plants, or from 
dislodged seagrasses and kelp. Some of this is carried well offshore in 
currents before becoming waterlogged and sinking. Sargassum is also a 
potential source of organic material in areas where it occurs (see 

Section 4.4). Larger plant particles sink rapidly enough to reach the seafloor 
more or less intact. Panels of wood placed at a depth of 1830 m and 
recovered 104 days later were riddled with burrows made by wood-boring 
clams, some of the very few animáis that can utilize wood for food. These 
bivalves convert woody plant material to foods that are available to other 
animáis. They produce faecal pellets that can be consumed by detritivores; 
those larval or adult clams that become exposed by the disintegration of the 
wood can be eaten by predators; and their dead remains become available 
for scavengers. Bacteria decompose and convert other types of macrophyte 
detritus into biomass available to benthic animáis. 

4. Animal migrations. The vertical migrations of zooplankton and fish 
result in a downward transfer of organic materials. Food that is captured in 
shallower depths is converted to animal biomass that may be consumed by 
predators at deeper levels, and faecal pellets may be released by migrators 
when they return to deeper water (see Section 4.5). Some deep-sea fish (e.g. 
angler-fish) spend their larval stages near the surface and then migrate intó 
the depths as juveniles or adults, where they become potential food for 
deep-sea predators. All of these events accelerate the pace at which food 
enters the deep sea. 

There is seasonal variability in the amount of organic material reaching the 
seafloor in températe and high latitudes due to seasonal differences in 
surface production. The sinking of large amounts of macrophyte detritus also 
may be linked to seasonal storms that dislodge seagrasses or trees. The 


dumping of trash fish also is largely restricted to seasons of relatively calm 
weather and availability of fish schools. Some fish die after spawning (e.g. 
mesopelagic blue whiting), and their carcasses deliver a seasonal signal to 
the underlying deep-sea fauna. 

Although some deep-sea species have seasonal reproduction, and growth 
bands in mollusc shells and echinoderm skeletal plates reveal seasonal 
growth in certain species, many deep-sea animáis have continuous 
reproduction and their production does not appear to be linked to seasonal 
surface events. Secondary production may be linked, however, to the 
sporadic nonseasonal occurrence of an adequate food supply. 

Although life in most of the deep sea is dependent on surface production, the 
deep-sea environment is spatially and temporally separated from the euphotic 
zone. In general, it is estimated that 75-95% of the organic matter in 
partióles sinking from the euphotic zone is decomposed and recycled in the 
upper 500-1000 m of the water column, above the permanent thermocline. 

In the Sargasso Sea, surface production is >100 mg C m~ 2 day -1 and the 
flux to the bottom at over 3000 m varíes from 18 to 60 mg m -2 day -1 
depending on season. However, organic matter constitutes only about 5% of 
the total sedimenting material, the remainder being mostly inorganic 
carbonate and silicate. In the north-east Atlantic, seasonally deposited 
phytodetritus has an even lower organic carbón content of less than 1.5%; 
however, deep-sea animáis have been observed to feed on this material 
despite its low nutrí ti ve valué. In general, only about 5-10% of organic 
matter produced in the euphotic zone will reach depths of 2000 to 3000 m, 
and progressively less in abyssal and hadal zones. Thus food is very scarce 
in the deep sea compared with other ocean regions. Food limitation is one of 
the most important factors goveming biological processes and community 
structure of the deep-sea benthos. 

CUESTION 8.12 Using the máximum sinking rales for diaioms (from 
Secíion 3.1.1), how long would il take a single dead Chaeíocews cefl to 
sink to 5700 m depth? 


Accumulating evidence suggests that various biological processes in 
deep-sea animáis, such as metabolism, growth, maturation and population 
increase, are slow in comparison to such processes in shallow-water 
environments. One of the first pieces of evidence resulted from an accident 
at sea. In 1968, the research submersible Alvin slipped from its launching 
eradle after the pilot and scientists were on board, but before the ports were 
secured. All three men managed to exit safely, but the Alvin sank in 1540 m 
of water and carried their packed lunches with it. The submersible was not 
recovered until over 10 months later, at which time it was discovered that 
the scientists’ lunches, although waterlogged, were still in good condition 
and edible. Placed for three weeks in a refrigerator at 3°C (the same 
temperature as at 1540 m), the food spoiled. This unexpected observation 
stimulated experiments in which organic substrates were exposed in situ at 
depths down to 5300 m; when these were recovered, they confirmed very 
low rates of bacterial metabolism in the abyss. The metabolic rate of abyssal 
bacteria living in sediments is from 10 to over 100 times slower than that of 
equivalent bacterial densities maintained in the dark, at the same low 
temperature, but at atmospheric pressure. Bacterial productivity is thought to 


range from about 0.2 g C m 3 day 1 (at 1000 m) to 0.002 g C m 3 day 1 
(at 5500 m). 

Low metabolic rates have also been reported for some benthopelagic 
animáis, including teleost fishes. However, studies of large deep-sea 
epibenthic decapods and echinoderms indícate respiration rates comparable 
to those of related forms in shallow water when measured at the same 
temperature. When oxygen uptake by benthic communities is compared, that 
in the deep sea is two to three orders of magnitude lower than that of 
shallow-water communities. This reduction in oxygen uptake is partly due to 
a lower density of organisms per unit area, but it also reflects the lower 
metabolic activity of deep-sea organisms. 

Another indication of slower biological processes carne from studies of 
Tindaria callistiformis , a small (<9 mm long) clam inhabiting soft sediments 
of the North Atlantic at 3800 m depth. Radioactive dating of the shells of 
this little deposit feeder suggested that it grows extremely slowly, with the 
shell increasing in length at about 0.084 mm yr -1 (see Figure 8.16 for a 
comparison with other molluscs). This would mean that Tindaria requires 50 
years to reach sexual maturity, and that its life span would be about 100 
years. However, the technique used to obtain these results has since been 
criticized, and faster growth rates have been estimated for other deep-sea 

The wood-boring clams (see Section 8.8.4) that rely on ephemeral falls of 
wood in the deep sea are notable exceptions to slow growth rates. In order to 
search out and colonize new sources of wood, they have evolved 
opportunistic life strategies that involve rapid growth, early maturity, and 
production of many young. 

QUESTION 8.13 Would you eonsider Tindaria caüistijonms and wood-boring 
clams to be priman ly exampies of r- or /¡f-seleeted s pee Íes? (Reler lo 
Section 1.3.1 and Table Lid 

Many deep-sea species have low fecundity. The number of eggs produced 
per individual is generally much lower in many deep-sea residents when 
compared with their shallow-water relatives; this can be related to the 
miniaturization that occurs in many groups. One small (< 1 mm long) 
deep-sea clam, Microgloma, produces only two eggs at a time, and even 
larger deep-sea clams may produce only a few hundred eggs at any one 
time. In contrast, shallow-water clams typically spawn tens or hundreds of 
thousands of eggs. Although type of development is not known for many 
deep-water species, the dominant mode appears to be production of 
lecithotrophic larvae. 

Low fecundity and therefore low dispersal suggest slow rates of 
recolonization in the deep sea, and this has been confirmed by experimental 
studies. Boxes of sterilized azoic sediment were placed at depths of 10 m 
and 1760 m, and examined after 2 months and after 26 months. The shallow 
boxes were colonized rapidly by invertebrates; after only 2 months, the 
boxes contained 47 species and 704 individuáis (35 714 individuáis m -2 ). In 
comparison, the deep-sea boxes yielded only 14 species and 43 individuáis 
(160 individuáis m“ 2 ) in the same period of time. Even after 26 months, 
recolonization of the deep-sea boxes was such that they contained 10 times 
fewer individuáis and species than samples taken from the surrounding 
sediment at the same depth. 


The low rates of metabolism and production shown by many deep-sea 
animáis can be correlated with low food supply in their environment, but it 
is also likely that the food requirements of deep-sea species are much lower 
than those of surface-dwelling animáis. Only a limited number of 
biochemical studies have been done on deep-sea animáis, but these suggest 
that body protein concentrations and caloric content decrease with depth in 
fish and crustaceans. On a weight for weight basis, the food requirements of 
a slow-moving rat-tail fish living in the deep sea are likely to be about 20 
times less than those of an active epipelagic salmón. Low rates of 
metabolism may also result from the physical-chemical constraints on 
enzyme kinetics that are known to occur at high pressures and low 


Much remains to be explored in the deep sea. Only a very small percentage 
of the seafloor has been examined using dredges, cores, submersibles, or 
remóte cameras. By 1995, less than 100 m 2 of the deep seafloor had been 
quantitatively sampled. Certainly we can expect that many more new species 
will eventually be discovered. More detailed physical studies, particularly of 
water flow over the seafloor, may further alter our perception of 
homogeneity in this environment. For example, the effect of abyssal storms 
on benthos is not known. 

We need to learn more about the biology of individual species in terms of 
their food and feeding patterns and energy requirements; their patterns of 
reproduction, development, and growth; and their interactions with other 
species. Only then can we approach an understanding of the factors that 
establish community structure, that determine production, and that maintain 
high species diversity in the deep sea. 


In 1977, scientists working off the Galápagos Islands discovered unusually 
high seawater temperatures at about 2500 m depth in an area where new 
seafloor is being formed. The hydrothermal activity accompanying this 
process manifests itself in the release of mineral-laden fluid either emitted as 
a warm (5-100°C) diffuse flow from cracks and crevices in the seafloor, or 
emerging as plumes of superheated (250-400°C) water from chimneylike 
vents. As the hydrothermal fluid mixes with the surrounding seawater, 
temperatures are moderated to between 8 o and 23°C. Chemical analyses of 
the water in the vent areas revealed low concentrations of oxygen but very 
high concentrations of hydrogen sulphide (H 2 S), a compound that is usually 
highly toxic to animáis even in much lower concentrations. 

QUESTIQN 8.14 What is the normal ambieni seawater temperature al u depth 
of 2500 m? Refer to Seelion 2.2.2. 

One of the most exciting events in benthic marine ecology occurred when 
scientists, diving in a submersible, discovered extremely dense concentrations 
of benthic animáis living in this hydrothermal vent area. Since that time, 
similar deep-sea communities of animáis have been found in other localities 
around the world, all of them in areas of tectonic activity. Sites at which 


biological investigations have been conducted inelude the Mid-Atlantic 
Ridge and spreading centres along the rim of the Pacific Ocean basin. 


Many of the animáis in the densely populated vent communities are of 
extraordinarily large size. The occurrence of very high benthic biomass in 
deep waters, far removed from surface photosynthetic production, 
immediately raised the question of how these animáis obtained sufficient 

Further studies revealed the existence of a food chain driven entirely by 
geothermal (terrestrial) energy, and not dependent on solar energy. Vent 
communities are dependent on the presence of hydrogen sulphide, a reduced 
sulphur compound that is released in hydrothermal fluid. This compound is 
utilized by sulphur-oxidizing bacteria (e.g. Thiomicrospira and Beggiatoa ), 
and the energy released by the oxidation is used to form organic matter from 
carbón dioxide by the same biochemical pathway that is employed by 
photosynthetic organisms. The reaction requires molecular oxygen which is 
provided by the surrounding seawater. The biochemistry of the 
chemosynthetic production can be generally summarized as: 

co 2 + h 2 s + o 2 + h 2 o -► ch 2 o + h 2 so 4 


In vent communities, chemosynthetic bacteria are the primary producers of 
the food chain, and the bacterial biomass becomes available for consumption 
by higher animáis. Mats of filamentous bacteria (up to 3 cm thick at some 
sites) can be grazed by animáis like limpets, and bacteria suspended in water 
can be filtered by suspensión feeders. In some cases, the bacterial production 
proceeds within tissues of host ánimals iri special symbiotic relationships. 

At present, the sulphur-oxidizing bacteria have received the most attention 
and are believed to comprise most of the bacterial biomass in vent areas. 
However, it is probable that other types of bacteria, utilizing different 
reduced materials (e.g. methane, ammonia) as sources of energy, also 
contribute to chemosynthetic production in these regions. In any event, 
bacterial production at hydrothermal vents is estimated to be two to three 
times that of photosynthetic production in the overlying water. 


Approximately 95% of the animáis discovered at hydrothermal vents have 
been previously unknown species. To date, about 375 new species have been 
described, many requiring the establishment of new taxonomic families 
because they are so different from related species. 

Spectacular giant, red, tube-dwelling worms found initially at the Galápagos 
vents proved to be a new genus and species, Riftia pachyptila (Colour 
Píate 39). These vestimentiferans (see Section 7.2.1 and Table 7.1) are 
encased in leathery tubes, with only a plume of many tentacle-like 
respiratory filaments protruding from the open end. They are highly unusual 
in lacking a mouth or digestive tract, but they are free-living and not 
parasitic. The largest vestimentiferans from the Galápagos site measure 
1.5 m long and 37 mm in diameter, and have tubes of up to 3 m in length. 
These animáis have extraordinarily high growth rates of up to 85 cm year -1 . 

Densities of Riftia can be as high as 176 individuáis m -2 , and biomass of 
Riftia alone ranges from 6800 to 9100 g wet weight m -2 . When combined 
with the wet weight of large bivalves living in the same site, biomass of this 
particular vent community can exceed 20-30 kg m -2 . 

QU ESI ION 8,15 How does the biomass of i he Galápagos vent community 
compare with typical biomass valúes at 2500 m deptli? Refer to Tabie 8.2. 

Riftia has a special internal organ known as a trophosome (meaning ‘feeding 
body’), which contains masses of symbiotic bacteria. These bacteria make up 
to 60% of the dry weight of a Riftia individual, and it becomes a semantic 
question as to whether this organism is more a bacterial colony than a worm. 
The haemoglobin of Riftia is unique in being able to carry both oxygen and 
hydrogen sulphide simultaneously. The bacteria obtain energy from 
hydrogen sulphide brought to them in the blood system of the worm; the 
bacteria utilize CO 2 and the energy derived from the oxidation of the 
sulphide to form organic carbón. Some of this organic carbón is, in some 
way, passed into the tissues of the worm. Whether this is the solé source of 
nutrition for these large worms, or whether they are also able to absorb 
dissolved organic matter (e.g. amino acids) from the surrounding seawater, 
remains unanswered. 

Another conspicuous and dominant animal at the Galápagos hydrothermal 
vents is a clam, Calyptogena magnifica , that reaches lengths of 30-40 cm 
(Colour Píate 40). The soft body parts of this bivalve are red, as they are in 
Riftia. In both animáis, the colour is derived from haemoglobin in the blood. 
Most molluscs contain the blood pigment haemocyanin; its replacement in 
Calyptogena by haemoglobin, a more efficient oxygen-carrier, may be an 
adaptation to the low and variable oxygen concentrations in the surrounding 
water. The gills of Calyptogena contain masses of attached sulphur bacteria 
and the clams, like Riftia , benefit nutritionally from this symbiotic 

Growth rates of Calyptogena have been calculated to be from 10 mm yr -1 
to as high as 60 mm yr -1 ; these rates are compared with growth rates of 
various other species of molluscs from different localities in Figure 8.16. 
Note that clams and mussels from vent areas have rates of growth that are 
comparable to those of shallow-water relatives, but they are approximately 
three orders of magnitude greater than those estimated for another deep-sea 
clam, Tindaria (see Section 8.8.5). Metabolic rates of Calyptogena and other 

Figure 8.16 Growth curves for different species of shallow- and deep-water clams and 


California Mussels 
Mytilus californianus 
West Coast, U.S.A. 

Giant Vent Clams 
Calyptogena magnifica 
_ (2600 m) 

Galápagos Vent Mussels 
Bathymodiofus thermophilus 

(2500 m) _ At | antjc Ribbed Mussels 
Geukensia demissa 
Virginia, U.S.A. 


Blue Mussels 
Mytilus eduiis 


Deep-sea Clams 
Tindaria callistiformis 
(3800 m) 

v lOOyears 

I I l I l l I I I 


large vent animáis are also similar to those of shallow-water relatives, and 
are orders of magnitude higher than those of related animáis in other parts of 
the deep sea. 

Related species of vestimentiferans and large clams similar to Calyptogena 
magnifica occur at other hydrothermal vent sites. Other vent molluscs 
inelude Bathymodiolus thermophilus , a giant mussel that has symbiotic 
bacteria on its gills but also is capable of suspensión feeding, and limpets 
(>30 species) and snails which graze directly on mats of free-living bacteria 
covering hard surfaces. Several types of suspension-feeding polychaete 
worms are associated with vents, including tube worms (Family Alvinellidae) 
that attach in large numbers to vents emitting superheated water. 

Deposit-feeding polychaetes are found in sediments around the vents. Long, 
thin enteropneusts, commonly known as spaghetti worms because of their 
appearance, can be abundant. Various types of crabs are found at most vents; 
some are scavengers, at least part of the time, and some prey on Riftia, small 
mussels, or polychaetes. Shrimp in densities of up to 1500 m -2 surround 
smoking vents on the Mid-Atlantic Ridge, and they apparently feed on 
bacterial mats. Sea anemones are abundant at certain sites, but other types of 
cnidarians are absent in these communities. A primitive type of barnacle 
dominates some Pacific hydrothermal localities. Fish are not usually 
important members of these communities; only five species ha ve so far been 
recorded from vents. 

Zooplankton are found in higher densities around vents than in surrounding 
waters. Copepods, amphipods and other planktonic crustaceans have been 
described, but few have been studied in detail. A new copepod genus 
(Stygiopontius ), with seventeen species, occurs at every vent site. Vent 
meiofauna is dominated by nematodes and benthic copepods, as it is in other 
deep-sea regions. As more hydrothermal vents are discovered and sampled, 
the list of new species of animáis grows rapidly. 


All of the explored hydrothermal vent sites are in waters deeper than 
1500 m, but geothermally-driven chemosynthetic production is not restricted 
to the deep sea. There are, for example, hydrothermal vents that release high 
concentrations of sulphides in intertidal areas off Southern California. There, 
benthic mats of sulphide-oxidizing bacteria contribute to the total primary 
production of the area along with photosynthetic production by benthic 
plants and phytoplankton. Limpets living near these vents are reported to 
graze on the bacterial mats, whereas limpets living in non-vent areas 
typically graze on photosynthetic algae encrusting rocks. 

In 1984 a community of exotic organisms was discovered in the Gulf of 
México, at the base of the Florida Escarpment. This massive limestone cliff 
rises some 2000 m above the sea bottom; at a depth of 3270 m, hypersaline 
waters containing high concentrations of sulphides and methane seep out 
onto the seafloor. Although the water temperature is low, the organisms in 
this coid sulphide seep area are remarkably like those found in hydrothermal 
vents. White bacterial mats cover exposed substrates. There are dense 
concentrations of 1-m-long tube worms (a new genus and species of 
vestimentiferan) as well as thick patches of large mussels and clams (the 
latter a new species of Calyptogena ). Snails, limpets, and crabs are also 
conspicuous inhabitants of this particular seep. 


Cold-water seeps result from a variety of causes, and they have been found 
along continental margins and in subduction zones where oceanic crust is 
carried back down into the Earth’s mantle. Those that have been explored 
support similar assemblages of animáis. Seep communities are also 
dependent upon chemosynthetic production by sulphide bacteria, and not on 
photosynthesis. The discovery of coid seeps demonstrates that the most 
important component necessary for high biological production in the deep 
sea is a source of reduced inorganic compounds, not heat. The existence of 
hydrothermal venís and coid seeps in deep waters indicates that low 
temperature and high pressure do not limit activities of deep-sea organisms. 
In deep areas where benthic ecology is dependent on photosynthetic 
production at the surface, biological processes and benthic production are 
limited by low food. In areas where sulphide-based food chains are possible, 
biological production in the deep sea may exceed that in the euphotic zone. 

Although each hot vent or coid seep site has distinctive physical features and 
distinctive fauna, the communities of animáis associated with high sulphide 
concentrations are similar in some respects. The dominant species are often 
ecologically similar, if not taxonomically related. Large vestimentiferans 
commonly occur at many sites, as well as similar species of limpets, clams, 
mussels, and crabs. All of the communities are characterized by having high 
population densities, high biomass, and rapid growth rates. These are unique 
concentrations of life in depths that usually are characterized by low density 
and low productivity; as such, these sulphide communities are appropriately 
referred to as ‘oases\ Bacterial chemosynthesis is the major source of food 
in all of these communities. No sunlight is necessary and no photosynthetic 
production is needed from the surface; the populations of organisms are 
sustained entirely by inorganic materials that are converted into bacterial 
biomass, which then becomes available for consumption by higher animáis. 


Although hydrothermal vents and coid seeps support communities with high 
biomass, species diversity is low compared to that at other deep-sea 
localities. Endemic species predominate; over 90% of the animáis found at 
vents and seeps do not occur outside their special habitats. The environments 
of vents and seeps possess certain attributes that place physiological 
constraints on animáis and require special adaptations. Many animal groups 
are not represented at these sites, presumably because they have not evolved 
the ability to cope with the special conditions. With a few exceptions, 
cnidarians other than sea anemones, echinoderms of all types, sponges, 
xenophyophores, brachiopods, bryozoans, and fish are uncommon or absent. 
Molluscs, polychaetes, and crustaceans account for more than 90% of all 
vent species. 

Hydrothermal vents, in particular, are transient environments that undergo 
large and rapid changes. The geological processes that create vents are 
dynamic events, and new vents are being formed as others cióse. Vent 
communities probably persist only for several years to several decades. Oíd, 
inactive vents are surrounded by the shell remains of clams and mussels that 
died when their energy source disappeared. 

Animáis living around vents are subjected to high temperature variance and 
to an oxygen concentration that can switch rapidly from anoxic to oxic 
conditions. There are also short-term fluctuations in H 2 S concentrations. 
Salinity in vent plumes varíes from about one-third to twice that of normal 


seawater. Hydrothermal fluid contains many inorganic substances which 
precipítate upon contact with seawater. The chimneys that emit superheated 
water are formed from the precipitation of sulphide deposits containing 
copper and zinc. This also means that animáis are subjected to a rain of 
inorganic precipitates that coat their surfaces, and that they are exposed to 
potentially toxic concentrations of dissolved and precipitated heavy metáis. 

Both vents and seeps contain very high concentrations of H 2 S (to 19.5 mM 
in hydrothermal fluid), and this raises the question of how the animáis 
escape being poisoned by the high levels of H 2 S. Hydrogen sulphide, even 
at concentrations less than one-thousandth of those found in some vent 
animáis, poisons aerobic respiration. Only certain bacteria have the 
appropriate enzyme systems to oxidize this molecule in order to obtain 
Chemical energy. Preliminary studies on Riftia indícate that it has special 
biochemical adaptations that protect the worm from H 2 S toxicity. These 
inelude a special sulphide-binding protein in its blood, and enzyme systems 
in its body wall that oxidize any free sulphide entering the cells. Other vent 
species may be similarly equipped with sulphide-detoxifying systems. 

Hydrothermal vent communities are quite small, usually only about 25-60 m 
in diameter, and they (as well as coid seeps) may be separated from other 
similar communities by as much as hundreds to thousands of kilometres. 
Vents are also ephemeral, lasting only on scales of years. 

Given these fealures, how can animáis mainlam populations in widespread 
local ¡lies and how are new vents populated? 

In order to succeed in short-lived, scattered habitats, animáis of these 
communities could be expected to grow rapidly to sexual maturity, produce 
many young, and have efficient means of dispersal (see Section 1.3,1). Such 
r-selected traits would allow them to reproduce within the time span of their 
habitat and to continually colonize new vent areas. Although reproductive 
studies of vent fauna are few, zoogeographic studies suggest that vent 
species do rely on larval dispersal. On-going research into types of larval 
development and dispersal mechanisms is seeking information on these 


1 Relative to most other marine habitats, intertidal areas are characterized 
by great fluctuations in environmental conditions. Littoral plants and animáis 
are specially adapted to cope with variable temperatures and salinity, and to 
withstand periodic exposure to air. 

2 Rocky intertidal regions support dense communities with a high 
proportion of epiflora and epifauna that may compete for limited space. 

Many of the sessile species are arranged in distinct vertical zones. The upper 
boundary of any particular zone is often set by physiological limits of the 
species, such as tolerance to desiccation and temperature change. The lower 
limits of zones are generally established by biological factors such as 
predation and competition. 

3 Intertidal areas of sand beaches support communities in which the 
primary producers are benthic species of diatoms, dinoflagellates, and 
blue-green bacteria, and the resident animáis are predominantly infauna and 


meiofauna. The meiofauna are specially adapted to live on sand grains, or in 
the interstitial spaces between sand partióles, by their small sizes, elongate 
shapes, protected integuments, and adhesive organs. 

4 Annual primary production averages about 100 g C m -2 in rocky 
intertidal areas, with a máximum of 1000 g C m -2 in particularly favourable 
areas. Benthic primary productivity in sand beaches is less than 

15 g C m -2 yr -1 , and this system relies on energy derived from detritus and 
from primary production in the surrounding water. 

5 Subtidal kelp forests occur on rocky substrates in coid températe regions. 
Kelp are among the fastest growing of any plants, and the productivity of 
kelp beds ranges from about 600 to more than 3000 g C m -2 yr -1 . Much of 
this production is not consumed directly, but enters the detritus food chain. 
Sea urchins are dominant components of kelp communities, and their 
feeding activities greatly influence the community structure. In some North 
Pacific kelp beds, otters are the top predators that act as keystone species. 

6 The circulation pattern of estuaries results in entrainment of nutrients and 
makes them some of the most productive of marine ecosystems. The upper 
reaches of estuaries are occupied by saltmarsh communities with total annual 
primary production generally ranging between 300 and >3000 g C m -2 . 
Seagrass beds typically form in intertidal areas of the middle reaches of 
estuaries, and total primary production by the seagrasses and associated 
epiphytes is about 600 to 1000 g C m -2 yr -1 . Both communities are 
dominated by detritus-based food chains. Estuaries also have subtidal 
mudflats or subtidal sand banks in which annual primary production (mostly 
by epipsammic algae) ranges from 10 to >200 g C m -2 . 

7 The high productivity of estuaries supports dense populations of animáis 
in some areas. However, many animáis are excluded from living in estuaries 
because of the fluctuating salinity, and thus species diversity is low. 

8 Coral reefs are formed by stony coráis that contain symbiotic 
dinoflagellates called zooxanthellae. The algae utilize carbón dioxide and 
waste products of the coral in photosynthesis, and in return the coral is 
provided with organic compounds such as glucose and glycerol. 
Photosynthetic fixation by the zooxanthellae provides only part of the energy 
required by coráis; the remainder is supplied by predation on zooplankton 
and bacteria, and by absorption of dissolved organic matter. 

9 Primary producers of coral reefs inelude phytoplankton, benthic algae, 
and zooxanthellae. Gross primary productivity is very high, ranging from 
about 1500 to 5000 g C m -2 yr -1 , but production to respiration ratios 
usually are between 1.0 and 2.5, and very little new nutrient material enters 
the system (i.e. /-ratio is <0.1). The high production of this system supports 
a community with very high species diversity. 

10 Coráis grow relatively slowly, at rates of <1 to 10 cm yr -1 . Growth of 
a reef is also controlled by bioerosion and physical events (e.g. storms) that 
destroy the carbonate framework. Net vertical upward growth of reefs varies 
from a few to almost 30 mm yr -1 under favourable conditions. 

11 Mangrove swamps occur along 60-75% of tropical and subtropical 
coasts. The major primary producers of these communities are salt-tolerant 
terrestrial plants that can live in oxygen-poor muddy substrates. The roots of 
the mangroves provide attachment sites for epifauna, and leaf fall is a major 


source of nutrients and energy for the detritus-based food chain. Net primary 
production is estimated to be between 350 and 500 g C m“ 2 yr _1 . 

12 The bathyal and abyssal zones together constitute over 90% of the 
benthic environment. Deposit-feeding infauna predomínate in 
organically-rich sediments, and the meiobenthos are particularly diverse. 
Benthic biomass diminishes rapidly with increasing depth, and annual 
secondary production is between 0.005 and 0.05 g C m~ 2 . Most areas of the 
deep sea depend upon the fall-out from production in the euphotic zone, but 
only a small proportion of sinking organic matter reaches the seafloor in 
depths over 2000 m, and food limitation greatly influences biological 
processes and community structure of the deep-sea benthos. In general, 
typical deep-sea inhabitants exhibit slow metabolic rates, slow growth rates, 
and low fecundity. 

13 Ocean trenches characteristically have a high proportion of endemic 
species. The biomass of hadal animáis ranges from about 0.008 g m -2 in 
trenches far from land and underlying oligotrophic water, to as much as 
9 g m -2 in trenches that lie near land under eutrophic water. 

14 Hydrothermal vents and coid seeps support unique communities that are 
independent of solar energy and photosynthesis. Instead, the food chain in 
these environments is based on the presence of hydrogen sulphide that is 
utilized by chemosynthetic bacteria to form organic compounds from carbón 
dioxide. The bacteria are the primary producers in these communities, and 
they are either consumed directly by animáis or they are found in symbiotic 
relationships with animáis. 

15 Deep-sea vents and seeps support extremely dense concentrations of 
large animáis, and biomass may be as much as 30 kg m~ 2 . Although these 
environments have plentiful food and, in the case of vents, temperatures that 
are higher than usual in deep water, relatively few animáis have developed 
the ability to live in high concentrations of H 2 S and species diversity is low. 

Now try the following questions to consolídate your understanding of this 

QUESTiDN 8.16 Apart from iheir high primary productivity, what important 
biological propeity do marshgrass and seagrass communities have in 
common that benefits animáis? 

QUESTION 8.17 If the re is intense compe t ilion for spaee in the coral reef 
ecosystem, can you think of any reason(s) why the fas test growing species 
dorTt overgrow or crowd oul other species and thus reduce the species 
diversity of the system? 

QUESTION 8.18 If sea leve] rose by 2 m in the next 100 years beca use of 
el i mate warming* would coral reef growth be able to keep up with the rise of 
Coastal waters? 

QUESTION 8.19 Although they are characterislic of differem latitudes, tropical 
mangrove swamps and températe saítmarshes share certain ecológica] 
fealures. What are these common characteristics? 

QUESTION 8.20 In lerms of quaniity, how important might crustacean moults 
he in ihe sinking of organic materials into the deep sea? Refer back to 
Seclion 4.2 for help with your answer. 

QUESTION fi.21 What are some of the biológica) or ecológica) advantages 
conferred on organismo ihal live at great depths? 

QUESIIQN 8.22 Of the two tvpes of marine food chains deseribed in the Jeep 
sea, one hased on bacterial ehemosymhesis and one on algal photosynthesis, 
vv h i c h wdu íd be evo] u ti o n ari 1 y o 1 d e r? i R e fe r to t he G eo I ogi c Ti i ne Seal e i n 
Appendix L) 

CUESTION 8.23 (a) Which henthic cotn muñí lies are characterized by having 

many endemic spedes? (b) What reason(s) explain the high degree of 
endemicity ín these comtnunities? 



Humans change marine environments and affect marine organisms in many 
ways. We harvest marine plañís and animáis, carry out mariculture, reclaim 
land, dam rivers that run to the sea, and dredge harbours. Marine organisms 
are transported around the world, and are deliberately or accidentally 
introduced into new areas. The sea has long been regarded as a convenient 
dumping site, and various pollutants are released into the marine 
environment from domestic or industrial outfalls or from accidental spills. 
Coastal ecosystems become enriched from nutrients contained in sewage, in 
discharged detergents, and in agricultural runoff. All of these activities, and 
others, may change the species composition of marine communities, result in 
loss of marine organisms or loss of marine habitats, or disrupt whole marine 
ecosystems. A summary of some examples of human impacts is given in 
Tables 9.1 and 9.2. 

Table 9.1 Major impacts of industrial activities on marine environments. 


Impact location 


Harvesting lisli 

World- wide 

Changes in the species 
composition of peí agio and 
benthic communities 

Changes in size structure of 
largeted fish populations 

Fishing methods 

World-wide (dependí ng 
apon specific types of 

Benthic trawling destroys 
bottom habitat 

Dynamite fishing destroys 

Unselective fishing meneases 
discarded by-catch 

Discard of by-catch 


Increase in scavenging species 

Accel eral ion in delivery of 
nutrients u> deeper water 

Possi ble mercases in benthic 

Dam construcción 

Rivers running lo sea 

Loss of habitat for anadromous 

Urban developtnent 

Estuaríes; coral reefs; 

Land reclamation leads to loss 
of habitat 

Sewage disposal and 
agricultural runoff may cause 

Industrial runoff may poIlute 
Coastal waters 

Com mere i a 1 sh i pp í n g 


Introducción of species into 
new environments 

Coral mining 

Tropical reefs 

Destrucción of coráis 



Whai human activity has caused ihe greaiest chances in ihc ocean? 

The greatest and most serious human impact on marine ecosystems is caused 
by the annual removal of more than 100 million tonnes of fish and shellfish 
(reported catch plus by-catch, see Section 6.7.1). This harvest affects the 
species composition of pelagic communities as well as nutrient 
concentrations in surface waters (see discussion of /-ratio, Section 5.5.1). 
Mid-water and benthic communities may also be impacted by the dumping 
of dead by-catch which delivers a rich source of nutrients to deeper waters. 
There are also disruptive habitat changes caused by bottom trawling. 

Advances in fishing technology have made it easier to lócate fish schools 
and to catch more fish more effectively. At the same time, the world fishing 
fleet has increased rapidly, doubling to about 1.2 million vessels between 
1970 and 1990. Long lines with thousands of baited hooks may extend more 
than 125 km from a ship, and some mid-water trawl nets with a mouth gape 
of 130 m and length of 1 km are large enough to encompass the Statue of 
Liberty or to extend around 12 jumbo jetliners. In some cases, more than 
80% of a commercially lucrative stock is removed each year. These facts are 
reflected in Figure 9.1 which shows the increase in marine fish catch from 
less than 20 x 10 6 tonnes in the late 1940s to about 85 x 10 6 tonnes in 1993; 

Figure 9.1 The marine fish catch (excluding by-catch) from 1947 to 1993, based on FA0 








these figures do not inelude by-catch. However, it has become increasingly 
clear that many fish stocks throughout the world are now dwindling and that 
the catch-per-unit-effort (CPUE) of fishing has decreased. In the last two 
decades, fish catches have declined in all the major oceans except the Indian 
Ocean, where modera fishing fleets only began operating intensively in the 
late 1980s. A report issued by the United Nations Food and Agriculture 
Organization in 1995 concluded that 70% of the ocean’s fish stocks are 
either fully exploited, overfished, or recovering from being overfished. 

Although ocean climate change may be responsible for declines and changes 
in some fish stocks (see Section 6.7.2), overfishing is clearly responsible for 
the declines in many commercially favoured species. The 200-year-old 
fishery for cod and haddock stocks off eastera Cañada and New England 
essentially ceased in the 1990s, although it may recover in time. Figure 9.2 
shows how declines in these preferred species have resulted in changes in 
the abundance of other fish; skate and dogfish populations have increased by 
35-40% since 1965, while cod, haddock, and hake have decreased by 45%. 
At the same time, less predation by cod and haddock on young lobsters has 
increased this lucrative shellfish harvest. Similar changes in community 
species composition have been documented in the North Sea, where 
sandlance greatly increased and became a target fish following declines in 
herring and mackerel stocks. In the Antarctic, commercial whaling and 
dramatic declines in the numbers of whales resulted in increased numbers of 

Figure 9.2 Changes in the relative abundance of different fish species on Georges Bank, 
U.S.A., between 1963 and 1992, following overfishing of cod, haddock, and hake stocks. 
(Data from the National Marine Fisheries Service, U.S.A.) 

cod, haddock 
and hake 




n i ®63 

EÜ3 1992 

other animáis that were dependent upon krill for food (see Table 5.2). In the 
1960s, commercial fisheries in the Black Sea targeted 26 species of fish, 
many of them large predators with long life cycles. Overfishing, construction 
of dams, and pollution have reduced this to only five commercially viable 
species, all of small size. On the other hand, the total biomass of the Black 
Sea fishery harvest has actually increased due to the greater abundance of 
smaller fish species whose populations are no longer kept in check by 
predation, and to increased fishing effort. Overfishing may also cause 
changes in size structure of fish populations; for example, the total weight of 
spawning Atlantic swordfish fell by 40% between 1978 and 1989. There are 
also examples of declines in shellfish populations due to a combination of 
overexploitation and Coastal pollution. Within 30 years, oyster catches in 
Chesapeake Bay, in the eastern U.S., fell from 20000 to 3000 tonnes. 

Anadromous fish, such as salmón, are adversely affected by the dams that 
block access to spawning areas. Despite expensive efforts to enhance stocks 
through hatchery rearing and construction of runways around dams, salmón 
stocks in the eastern North Pacific Ocean have declined in some rivers (e.g. 
the Columbia River). Loss of Coastal spawning grounds and nursery areas 
through land development and/or pollution is another increasing problem for 
many fish species. Fishing activities may also destroy habitats. Heavy fish 
trawls can penétrate 6 cm or more into the seabed, thereby disrupting the 
natural substrate and releasing nutrients into the water column, and 
destroying zoobenthos that may be food for the demersal fish stocks. In 
particularly rich fishing grounds, more than 70% of the sediment can be 
ploughed by trawls. 

Commercial fisheries discard about one of every four animáis caught; the 
percentage of this unwanted by-catch may in fact be larger because much 
by-catch goes unreported. The discarded catch ineludes species with no 
economic valué and young fish that are too small to market. In some cases, 
the by-catch may exceed the target catch. Shrimp fisheries have an 
exceptionally large by-catch. The shrimp fishery in the Gulf of México 
catches and discards at least 5 million juvenile red snapper annually, or 
4.2 kg of fish for every 1 kg of shrimp. World-wide, the total by-catch of 
shrimp fisheries may be as much as 17 million tonnes per year; much of this 
is made up of small fin-fishes, most of which do not survive capture and 
release. Discarded by-catch increases the number of scavengers in fishing 
grounds. In the North Sea, it is estimated that the annual discarded by-catch 
of about 90000 tonnes of whitefish can potentially support about 
600000 gulls. 

Unfortunately, as a valuable fishery species becomes more scarce, its 
economic valué tends to increase. Thus it often remains profitable for fleets 
to continué to take an overfished species. Spawning populations of bluefin 
tuna (the world’s most valuable fish) have declined by about 80% in the 
western Atlantic since 1970, and by 90% in the Gulf of México since 1975, 
but a single large specimen may fetch more than US $80 000 (about 
$265 per kg in 1996) on the market. 

Declining world fish catches have alerted nations to the fact that it is indeed 
possible for man to deplete fish populations and to alter oceanic ecosystems 
over vast regions. Some progress has been made to alleviate the problems of 
overfishing; regulations have been set concerning allowable sizes and total 
catch for some species, driftnets have been largely banned, tuna fishers have 
adopted new methods to avoid capturing and killing dolphins, and whales 


are no longer the targets of commercial fishing. But International regulations 
are difficult to establish and to enforce on the open seas, and economic 
issues, not scientific management, continué to drive the industry. Hopefully 
the next decade will see the resolution of global fisheries problems before 
entire fish stocks and pelagic ecosystems are irrevocably changed. 


Marine pollution has been defined by the Intergovemmental Oceanographic 
Commission as the introduction by humans, directly or indirectly, of 
substances or energy sources into the marine environment resulting in 
deleterious effects such as harm to living resources; hazards to human 
health; hindrance to marine activities, including fishing; impairment of the 
quality of seawater; and reduction of amenities. The number of different 
pollutants entering the sea is very large, and new substances are added every 
day. Some of the substances regarded as pollutants, like heavy metáis and 
Petroleum hydrocarbons, occur naturally in the sea and human introductions 
add to natural concentrations. ¡Some introduced pollutants will decompose in 
time or will be attenuated by the very large volume of the oceans, so that 
their effect will not be noticeable. Other pollutants may have significant 
impacts. Some of the major anthropogenic pollutants and their effects are 
summarized in Table 9.2 and discussed.below. 

Table 9.2 Some major forms of marine pollution and their effects. 

Poli mam 

Local ion 




Local otl spills 

Mass mortality of benlhos and 

World-wide seas 

Lo w- l e ve 1 concentrati on efTects 



Aeslhetically dísturbing 

Floating debris 

Entanglement of animáis; 
ingestión by animáis 

Pesticides and 
related compounds 

Local point-source inputs 

Acute toxicity 

World-wíde seas 

Long-term sublethal effects 
largely unknown 

Heavy metáis 

Industrial outfalls 

Mostly sublethal effects 
catising growth abnormalities 

Se w age 

Local outfalls; 
agri cultural runoff 

Eutrophication and allcrcd 
community strocture; 
introduction of pathogens 

Radioactivo w as tes 

Local power plants; 
historical at-sea dumping 

General ly considered to be 
helow ha mi ful le veis 

Thermal effluents 

Local power plants 

Warming leads to a he red 
co m m u n i ty si ruct u re 



Petroleum hydrocarbons have probably attracted the most attention as marine 
pollutants because the impact of an oil spill is visually very apparent. 

Table 9.3 lists some of the major spills, the largest having occurred during 
the Arabian Gulf war when approximately one million tonnes of oil were 
spilled into the Gulf of Arabia. The largest spill from an oil tanker occurred 
when the Amoco Cádiz went aground off Brittany in 1978, releasing 220000 
tonnes of crude oil (Colour Píate 41). More than 300 km of shoreline were 
affected, causing the elimination of at least 30% of the marine benthic fauna 
and the death of some 20000 birds. The effects of the Exxon Valdez spill 
(ca. 30 000 tonnes of oil) on Alaskan populations of birds and otters were 
noted in Sections 6.5 and 8.3. Although such large spills are devastating 
within localized areas, the natural recovery time of shoreline communities, 
under modérate conditions of wave action, is usually within 5 to 10 years for 
most organisms, although bird and otter populations may take longer to 
recover because of their slower reproductive rates. 

Table 9.3 Some major oil spills in the ocean. (Italicized ñames are of oil tankers.) 



Amuunt of 
oil spilled 


Arabian Gulf 

Gulf War 

1 000000 


Gulf of México 

Oil well 

440 (XX) 


Brittany. France 

Amoco Cádiz 



Comwall, U.K, 

Torrey Canyon 



Wales. U.K. 

Sea Empress 

70 (XX) 


Japan Inland Sea 

Storage tank 



Alaska, U.S 

Exxon Valdez 

37 000 


Northwest Atlantic 

Argo Merchartt 



North Sea 

Oil well 

15 (XX) 


Tanker accidents are responsible for only a small percentage of the oil 
entering the sea. The production and transportation of oil, conventional 
shipping, waste disposal, and runoff are all additional sources of oil in the 
marine environment. There are also natural seeps, where oil deposits cióse to 
the Earth’s surface leak into the sea. It is estimated that between 2.5 and 5 
million tonnes of petroleum hydrocarbons enter the ocean each year from all 
sources. Over time, very small amounts (ppb) of petroleum hydrocarbons 
have accumulated world-wide in the oceans. The experimental toxic effect of 
such hydrocarbons is generally at concentrations of parts per million (ppm), 
and therefore the present accumulated background is not considered harmful 
to marine organisms, although effects of long-term chronic exposure to such 
concentrations are not fully known. 

QUESTIDN 9.1 In wbat type of marine environment would you ex pee t to tind 
the ¡dowest rale of recovery from a. large oil spill? 


Discarded plástic materials in the oceans range in size from large nylon drift 
nets (see Section 6.2) to pellets of less than a millimetre in diameter which 
can be distributed by the wind over the whole ocean. These materials are not 
biodegradable; although plastics do break down as a result of physical and 


Chemical weathering, this process is slow and therefore plastics accumulate 
over time in the sea. Loose driftnets (or ‘ghost nets’) or other discarded 
fishing gear, for example, can continué to entangle marine animáis for years 
before washing ashore or sinking. Plástic bags or small plástic pellets are 
often mistaken for prey and ingested by marine turtles and seabirds, 
respectively. Pellets have been found in at least 50 species of marine birds. 
A survey of shearwaters (Pujfinus sp.) in the North Pacific Ocean revealed 
that more than 80% of 450 birds had plástic partióles in their stomachs. 
Ingested plástic bags are known to kill turtles; although one can speculate 
that ingested pellets may be harmful to birds or other marine life, there is 
presently no direct evidence to support this. 

QUESTIQi 9.2 Can yon think of any ways lo reduce the amoum of plástic 
material that entera the sea eaeh year? 


The most common pesticides entering the oceans are various forms of 
chlorinated hydrocarbons. These man-made compounds do not occur 
naturally, they are not readily degraded by Chemical oxidation or by bacterial 
action, and they accumulate in animal fat tissues because they are 

The best-known insecticide, DDT, was first employed in 1940, and within 20 
years it and its residues could be found throughout the biosphere. Because 
DDT was often sprayed from aircraft, it was easily carried by winds into the 
oceans. Eventually even Antarctic penguins, living several thousand 
kilometres from any place where DDT had been used, were found to contain 
ppb traces of DDT. During the 1960s, there was increasing evidence that 
marine organisms, particularly seabirds, were being adversely affected in 
marine areas where DDT concentrations were exceptionally high. One 
example occurred off Southern California, where a pesticide company had 
released DDT for 20 years into the coastal environment. DDT entered the 
ocean food chain, and its effects on the marine biota could be detected for 
100 km along the coast. Fish in this area contained >3 ppm DDT, and 
pelicans and sea lions that fed on the fish accumulated even higher tissue 
concentrations of DDT and were unable to breed successfully. Even 
following a dramatic reduction in DDT emissions after 1971, DDT levels in 
fish remained high for years. Harmful effects were found in the Los Angeles 
Zoo in 1976, with the death of all the cormorants and gulls that had been fed 
inadvertently for several years on locally caught fish contaminated with 
DDT. Autopsy results found DDT concentrations ranging between 750 and 
3100 ppm in liver tissues of these birds. 

DDT usage has now been banned in some countries, but continúes to be 
used in tropical areas as an effective control against mosquito populations 
that carry malaria. At present, most of the chlorinated hydrocarbon in the 
sea, and 80% of that in marine organisms, is in the form of DDE, a Chemical 
derived from the breakdown of DDT. In most surface waters, DDT/DDE 
concentrations are between 0.1 and 1 ng l -1 (or less than one part per 
trillion), and these levels are not considered to be harmful. It is now 
recognized that heavy use of synthetic pesticides in agriculture is associated 
with significant undesirable side effects, and efforts are being made to find 
alternatives through biological or genetic Controls of pests. 


In addition to pesdcides, several toxic chlorinated hydrocarbons are used 
industrially and may be present in seawater. These inelude dioxins and PCBs 
(polychlorinated biphenyls), both of which may have deleterious effeets on 
marine life. PCBs are stable compounds that tend to persist in the 
environment and to be concentrated through biological processes. These 
characteristics were underscored by the discovery of exceptionally high PCB 
concentrations in several beluga whales that died in the St. Lawrence River 
in 1985; they contained up to 575 ppm in lipid tissue and 1750 ppm in the 
milk. The recognition of environmental problems has resulted in a ban on 
production and usage of PCBs in the United States. In the mid-1960s, the 
organo-tin compound tributyl tin (TBT) was found to have exceptional 
antifouling properties, and was consequently applied to boat hulls and 
fishing nets to prevent the settlement and growth of marine organisms. 
Unfortunately it leaches into surrounding waters, where concentrations 
between 0.1 and 100 /xg l -1 are toxic to the larvae of many benthic 
invertebrates, and levels as low as 0.001 /xg l -1 may affect reproduction in 
some marine snails. Ship traffic has spread TBT globally and the compound 
has accumulated in sediments near harbours and ports. Several countries 
have now established regulations designed to curb TBT usage. 


Heavy metáis such as mercury, copper, and cadmium occur naturally in 
seawater at low concentrations, and they enter the sea through natural 
erosión of ore-bearing rocks and subsequent transpon in rivers or via dust 
particles in the atmosphere, and through volcanic activity. All of these metáis 
can be poisonous to organisms in high concentrations, and thus potential 
health problems exist where heavy metáis accumulate in the sea around 
industrial outfalls, or at marine sites used to dispose of some types of mine 
tailings. At such localities, the local benthos may accumulate metáis in levels 
exceeding permitted concentrations for marketable marine producís (the 
permitted level for mercury is 1 ppm). Generally, heavy metáis have acute 
toxic effeets, but accumulations of these substances in marine animáis may 
also cause chronic effeets such as growth abnormalities, including cancers. 

A serious case of heavy metal poisoning in humans occurred in Minamata, 
Japan, where a plastics factory discharged an estimated 200-600 tonnes of 
mercury over a period of 36 years into the local bay. Illnesses began to 
appear in the early 1950s, and by 1956 these were diagnosed as mercury 
poisoning derived from eating contaminated shellfish and fish from the bay. 
Effeets included severe neurological damage, paralysis, and birth 
deformities. By 1988, 2209 victims had been verified, of whom 730 died. 
Following this tragic discovery of the dangers to human health from eating 
mercury-containing seafood, regulations were adopted in many countries to 
limit mercury discharges and to limit the tissue concentrations allowed in 
seafood producís. 

What are normal concentrations of mercury in the muscle tissue of pelagic 

Most species of fish in uncontaminated oceanic waters contain about 
150 /xg kg -1 (0.15 ppm) of mercury in their muscles. However, some large 
pelagic species such as sharks, swordfish, black marlin, and tuna may have 
tissue concentrations as high as 1-5 ppm, but these levels are not indicative 
of anthropogenic pollution. These long-lived fish are large carnivores at the 


end of food chains, and their high mercury levels are acquired by 
bioaccumulation over their life spans. 

Although other heavy metáis, such as cadmium, copper, and lead, may 
accumulate in marine organisms exposed to high concentrations at waste 
discharge sites, they are not known to have caused serious human health 
issues. Problems arising from the use of another metal, tin, have been 
discussed in Section 9.2.3. International agreements now regúlate marine 
discharge and usage of some of the more dangerous metáis. 

9.2.5 SEWAGE 

Sewage disposal is a major form of Coastal pollution throughout the world. 
Sewage outfalls near Coastal communities release human waste as well as 
other organic matter, heavy metáis, pesticides, detergents, and petroleum 
producís. Nutrients from organic waste material may cause eutrophication; 
local waters may also be nutrient-enriched by detergents that contain 
phosphate and by agricultural and horticultural producís entering from 
runoff. In addition, human sewage delivers pathogenic bacteria and viruses 
that are not necessarily killed by exposure to seawater; high concentrations 
of these microbes make local seafood unsafe to eat and contaminated waters 
unsafe for bathing. The chief health risk from sewage is through eating 
contaminated seafood, particularly filter-feeding clams or mussels which 
accumulate human pathogens on their gills. The cholera virus is a particular 
problem in some countries, and may be transmitted in just such a manner. 

In urban areas of developed countries, sewage may receive special treatment 
to degrade organic matter or to remove nitrates and phosphates, but these 
processes are expensive. Usually no more sewage is treated than is deemed 
necessary, and in many places sewage is released into the sea without 
treatment. Generally the immediate area (within 100 m) around a large 
sewage outfall may be anoxic and dominated by anaerobic bacteria. At some 
greater distance from the outfall (within several km), nutrient enrichment 
typically leads to increased production of green macroalgae (Enteromorpha 
or JJlvá) that form thick mats along the shoreline. A few opportunistic 
animáis, such as the polychaete Capitella , are also indicative of sewage 
enrichment and may dominate affected benthic communities. At some ten 
kilometres from a major domestic outfall, there is usually sufficient 
attenuation of pollutants that community species diversity is not affected. 


Radioactive wastes enter seawater from nuclear testing, from nuclear power 
plants or reprocessing reactors, or from delibérate dumping of waste 
materials. Heavy radionuclides have low solubility in water and tend to be 
adsorbed onto particulate matter; they therefore accumulate in sediments. 
Isotopes with long half-lives (e.g. caesium-137, strontium-90, and 
plutonium-239) are especially hazardous and are usually monitored in areas 
where they may escape from nuclear facilities. Barring major accidents, 
background levels in the marine environment around radioactive outfalls are 
generally regarded as safe. The potential for reaching high concentrations of 
radioactive materials exists in certain localized areas, notably around the 
several known sunken nuclear submarines, from nuclear dump sites at sea 
(which are now prohibited), and from nuclear testing that has been carried 
out within coral atolls (most recently in the South Pacific by France). 
However, it is predicted that leakage from such sources would occur at a 

slow rate and that there would be dilution of soluble radionuclides and 
adsorption of others on to bottom sediments. 

Some marine organisms (e.g. seaweeds and bivalves) may accumulate 
radionuclides from surrounding water. For example, the alga Porphyra 
umbilicalis , growing in the vicinity of a reprocessing plant in England, 
accumulated 10 times the concentration of caesium-137 found in the ambient 
water and 1500 times the concentration of ruthenium-106. The experimental 
consequences of low-level doses of radiation on marine organisms are the 
same as those for terrestrial species and may inelude increased incidences of 
cancers, impaired immune systems, and genetic defeets causing growth 
deformities. However, present levels of radiation in the sea have not 
produced any measurable environmental impact on marine biota. 


Power plants may discharge several hundred thousand cubic metres of 
cooling water per hour into Coastal waters, and this thermal effluent may 
raise the local seawater temperature by 1-5 C°. In some areas, this warmed 
water can be used beneficially to enhance growth rates of organisms grown 
in mariculture. However, in mañy cases elevated temperatures cause 
unwanted changes in the natural fauna and flora. For example, a persistent 
elevation of the ambient temperature by 5 C° along the subtropical Florida 
coast resulted in the replacement of natural algae and seagrasses by mats of 
cyanobacteria. In another example, the increase in water temperature of a 
températe area of the U.S. allowed the entry of warm water wood-boring 
bivalves which caused damage to boats and wharves. In most cases, the area 
affected is limited to the plume of hot water and its immediate surroundings, 
an area that may range from less than one hectare to about 40 hectares. 

There may be other effeets from thermal effluents. They often contain 
chlorine, which is added to intake water to prevent fouling organisms from 
blocking pipes, and as little as 0.1 ppm of chlorine remaining in the effluent 
can be toxic to some organisms. Water flow of the plume also mechanically 
scours the seabed and so influences the fauna. 

QUEST10N 9.3 Ai what tíme of the year would yuu expeci to have the 
greatesl impact from thermal effluents released from power plañís Incaled on 
a subtropical coast? 


The movement of species from one región to another occurs naturally 
through larval drift, rafting, and other means, but humans have accelerated 
these movements and eliminated natural ocean barriers by accidental or 
delibérate introductions of species into new areas. In many cases, introduced 
species fail to develop reproducing populations, but in some instances an 
exotic species encounters favourable conditions and causes a significant 
impact on its new environment. 

Marine organisms have often been introduced deliberately into new 
environments for mariculture purposes. For example, the Japanese oyster 
(Crassostrea gigas ) and the east coast oyster (C. virginica ) were brought to 


the north-west coast of North America because they were larger and grew 
more rapidly than the local oyster ( Ostrea lurida)\ simultaneously, predatory 
snails that bore into oysters and other molluscs were accidentally introduced 
and both the oysters and this predator are now firmly established in 
this región. 

Increased demand from the mariculture and aquaria industries and increased 
ship traffic have accelerated the rate at which species are transported to new 
environments. Each day, approximately 3000 species of marine animáis and 
plants are being carried across oceans in the ballast tanks of ships that take 
on seawater in port and then release it at later ports of cali. Examination of 
ballast water released in Oregon from a Japanese vessel contained 367 taxa 
of both holoplankton and meroplankton. Thus entire planktonic communities, 
including the larvae of benthic organisms, can be transported across natural 
oceanic barriers. 

The release of the ctenophore Mnemiopsis leidyi (see Section 4.7) into the 
Black Sea is one example of an invasive species that encountered favourable 
conditions and no natural predators, and rapidly became so abundant in the 
Azov and Black Sea that it caused drastic declines in the resident 
zooplankton and subsequent declines in the anchovy fisheries of bordering 
countries. More recently, two species of jellyfish that are native to the Black 
Sea have appeared in San Francisco Bay (Figure 9.3); it is too early to 
assess their impaets on the ecosystem. Asian species of copepods are now 
present in Californian harbours, and the Chinese clam Potamocorbula 
amurensis has become one of the most abundant marine animáis in the 
estuary of San Francisco Bay. The zebra mussel Dreissena polymorpha , 
originally from the Mediterranean, entered the North American Great Lakes 
from ballast water in 1986 and almost immediately began to grow in such 

Figure 9.3 Maeotias inexspectata, a jellyfish native to the Black Sea that has recently been 
introduced into San Francisco Bay, California. 

profusión that it blocked water intakes, resulting in an expense of ten billion 
dollars to deal with its destructive effects. 

Australia has had over 35 successful invasions in recent years; the predatory 
starfish Asterias amurensis has invaded the coast of south-eastern Australia, 
and a common Japanese kelp, Undaria pinnatifida , is spreading along parts 
of Australia at a rate of 55 km a year. It is believed that the appearance of 
red tides (see Section 3.1.2) for the first time in Tasmanian waters resulted 
from toxic dinoflagellates being transponed in ballast water of cargo ships; 
one such vessel contained more than 300 million toxic dinoflagellate cysts in 
the sediment accumulated in its tanks. 

How could the spread of marine organisms by transpon in hall asi water be 
red ti c e d ? 

Ships take on ballast water in Coastal seaports. The problem of global 
transport of marine species could be ameliorated by requiring ships to 
exchange ballast water at sea. Coastal organisms are unlikely to survive in 
oceanic water, and oceanic organisms are much less likely to survive release 
into low salinity Coastal water. 



Many of the world’s largest cities are located on estuaries; London, 
Shanghai, and New York are but a few examples. It is not surprising, 
therefore, that estuaries have suffered more from human impact than most 
other marine environments (see Figure 8.6). The most severely affected area 
is usually the saltmarsh community because this upper intertidal area is 
easily reclaimed for housing and industrial activity and, sometimes, for 
airport construction. In some localities, more than 90% of this community 
has disappeared. The deeper seagrass and mudflat communities are often 
disrupted by dredging operations to make large harbours and deepen 
shipping channels. 

The same features that make estuaries productive also make them especially 
vulnerable to pollution. Just as nutrients are retained within the System (see 
Section 8.5), so are pollutants like petroleum byproducts, heavy metáis, 
fertilizers, and pesticides. Both the plankton and benthic communities are 
affected by domestic and industrial outfalls releasing organic pollutants that 
may cause unwanted eutrophication. Pathogenic organisms, heavy metáis, 
and pesticides that enter estuaries may all eventually work their way up the 
food chain into edible producís for humans. 

Because estuaries are naturally productive, they are often favoured fishing 
grounds and are frequently used for harvesting shellfish or for developing 
mariculture. Recently, however, many estuarine fisheries and shellfish beds 
have been closed due to high numbers of coliform bacteria from domestic 
sewage, or from the accumulation of pesticides or heavy metáis in fish 
producís. All of these forms of pollution may also alter the structure of the 
estuarine ecosystem so that traditional spawning grounds and nursery areas 
for fish may be lost. In extreme cases of pollution and eutrophication, anoxic 
zones may occur where only bacteria can survive. 


Because of the dangers of human health risks from pollution of estuarine 
waters, monitoring programmes have been established in developed 
countries. Many pollutants occur in very low concentrations (ppb level) in 
seawater and are therefore extremely difficult to measure, even using 
sophisticated equipment and techniques. For this reason, a special programme 
called Mussel Watch was established to monitor concentrations of marine 
pollutants in mussels. This programme uses mussels ( Mytilus) because they 
are abundant world-wide in Coastal regions, because they are sessile animáis 
that are exposed to any pollutants contained in the water flowing over them, 
and because these filter-feeding bivalves are known to accumulate a variety 
of pollutants. They have also been extensively studied, both experimentally 
and ecologically. Tissue levels of pollutants that have accumulated in 
mussels from contaminated areas can be more easily measured than pollutant 
concentrations in ambient waters, and these valúes can be compared with 
standards in mussels from uncontaminated regions. Since 1986 in the United 
States, mussels (or oysters) have been collected from over 200 localities 
once each year. Tissues are analysed for several heavy metáis, chlorinated 
hydrocarbons including DDT and PCBs, and tributyl tin. Analysing these 
pollutants in mussel tissues is much easier and cheaper than analysing the 
same substances in trace concentrations in seawater. 

QUESTIOfi 9.4 Drawing on your general knowledge of geography, can yon 
think of any large estuarios in the world that do noi have adjacem cutes and 
are not affected by human populations? 


Mangrove swamps (see Section 8.7) suffer from many of the same 
environmental disturbances that are experienced by estuaries. Dredging, land 
reclamation, garbage and sewage dumping are all disturbances that can have 
significant impacts on mangroves near populated areas. In these tropical and 
subtropical ecosystems, insect control (particularly of malaria-carrying 
mosquitoes) has resulted in accumulations of pesticides in estuarine 
sediments and in mangrove food chains. During the Vietnam war, spraying 
of herbicides on mangrove swamps defoliated and destroyed as much as 
100000 hectares. Oil spills smother both algae and invertebrates, and disrupt 
the oxygen supply to the root system. Where river water has been diverted 
into irrigation systems, the reduction in freshwater discharge and the 
resulting elevated salinities may be detrimental; for example, a considerable 
area of mangrove swamp has been destroyed by diversión of water flow 
from the Indus River in Pakistán. 

Overcutting of mangroves is, and has been for centuries, a serious problem 
in many areas. Mangroves once existed along the shores of the Persian Gulf, 
where they were a much-needed source of firewood for humans and of green 
fodder for camels in a desert environment, but they were eventually 
eliminated by overcutting. Some efforts had been made to re-establish 
mangroves along north-eastern Saudi Arabia, but these were destroyed by 
the Gulf War. Other countries, recognizing the benefits of mangroves, have 
also developed afforestation programs, reintroducing mangroves with varying 
degrees of success. Globally, however, destruction of mangroves is 
progressing faster than reintroduction. Almost half of the world’s mangroves 
have been eliminated in recent years in order to build shrimp farms or rice 
paddies. In countries like Bangladesh, removal of this buffering zone has led 
to intensified coastline inundation and erosión from tropical storms. 


Typhoons and hurricanes remain perhaps the greatest destructive agents of 
mangrove swamps, as they affect very large areas and occur frequently. Not 
only do they uproot trees, but severe storms alter the salinity of both water 
and soil, and they cause massive sedimentation. It is estimated that recovery 
of mangrove forests from very violent storms takes at least 20 to 25 years. 
Whereas little can be done to reduce damage from natural events, it is 
possible to develop management policies for the exploitation of mangrove 
resources, including replanting. The rational utilization of mangrove areas 
depends ultimately on increasing public awareness of the importance of this 
unique marine community to local populations in developing tropical 


Coral reefs not only have great beauty and support a very high natural 
diversity with many endemic species, but they also have wider biological 
and economic importance. Because coráis remove large amounts of bound 
CO 2 from the oceans during calcification, the reefs play a role in the global 
CO 2 budget. They are of benefit in protecting coastlines and providing 
sheltered harbours. And as air travel has become cheaper and more 
available, reefs have brought in more and more Tourist dollars’ to boost the 
economy of human populations living in their vicinity. 

However, coral reefs are extremely vulnerable to disturbance, and reefs are 
presently regarded as declining. The World Conservation Union and the 
United Nations Environment Program (UNEP) reports damage or destruction 
of signiñcant amounts of reefs in 93 out of 109 countries. Much of this 
destruction results from human activities, some from changes in ocean 

Expanding human populations near reefs often result in the addition of 
various types of pollutants to near-shore waters. These can inelude 
agricultural runoff, pesticides, industrial pollutants, and sewage from 
beachfront hotels or Coastal communities. Many of these sources increase the 
nutrient concentrations in the seawater, and this eutrophication triggers 
outbursts of benthic algae that can outeompete coráis for available space. 
Often the algae overgrow the coráis, smothering and killing the reefs by 
cutting off the sunlight required by the zooxanthellae. Just such an event 
followed a burst of land construction and development in an area of the 
Hawaiian Islands. After thick mats of algae overgrew and killed large areas 
of the reef, the decomposition activities of bacteria led to a lowering of 
oxygen concentrations in the seawater. The final result was a dramatic 
decrease in diversity, with a particular sea cucumber becoming the 
dominant animal. 

Various types of Coastal development also result in increased land erosión, 
which then increases the amount of sediment in water overlying the reef. 

The suspended silt decreases light penetration, thus reducing photosynthesis 
of the zooxanthellae and diminishing a nutritional source for the coráis. 
Although coral polyps are capable of removing some settling sediment, 
using mucus trapping and cilia to cleanse their surfaces, excessive quantities 
of silt will clog this apparatus and smother the polyps. Deforestation, leading 
to increased runoff and excessive sedimentation, is a major cause of coral 
reef destruction. Logging in one area of the Philippines (Bacuit Bay) has 
increased erosión and killed 5% of the area’s reefs. Dredging to deepen 


harbours or open ship channels through the reef has similar effects on 
adjacent reefs. 

A coral reef offers a number of resources used directly by humans. In some 
localities, coral is cut and used as a favoured building material (Figure 9.4). 
In the Maldive Islands (Indian Ocean), about 200000 m 3 of coral rock have 
been mined in the past 20 years; this represents about one-third of the 
available coral in shallow water. This practice has also destroyed large areas 
of reefs in French Polynesia and Thailand. 

Local inhabitants have traditionally relied on fish from nearby reefs as an 
abundant protein source, but demands have increased for this resource as 
populations have grown in size and as expanding numbers of tourists enter 
the communities. At the same time, private and public aquaria have 
increased in number throughout the world, and the capture of exotic ñsh for 
sale to the aquarium trade has become an extremely lucrative business, 
worth about $40 million per year (in 1996). To meet these increased 
demands, traditional fishing methods often have been replaced by much 
more detrimental techniques. It has become common to blast with dynamite 
to stun fish, which are then easily collected when they rise to the surface. At 
the same time, portions of the reef are destroyed, and it is estimated that it 
may take some 40 years for areas destroyed by blast fishing to recover to 
50% live coral cover. Cyanide is also used to stun fish for live collection, 
although it may result in the death of the fish and certainly kills other reef 
species. In the Philippines alone, an estimated 150 tonnes of sodium cyanide 
is used annually for fish capture. Increased fishing, whether by traditional or 
non-traditional methods, has resulted in overexploitation of many species in 
many regions. Where destructive fishing techniques have been banned, 
regulations often are not enforced. 

Fish are not the only reef inhabitants that are removed for consumption or 
trade. Coráis are removed for ornamental purposes; about 1500 tonnes of 
such coral was imported into the United States alone in 1988. Spiny lobsters, 

Figure 9.4 Mined coral that has been cut for building purposes in the Maldive Islands, 
Indian Ocean. 


sea cucumbers, and sea urchins are among some of the reef animáis that are 
considered delicacies in many parts of the world. Snails and clams are 
collected for food, or to sell to tourists and shell collectors. Remo val of large 
numbers of animáis from reefs may alter the ecology. For example, sea 
urchins are responsible for removing part of the reef framework during 
grazing; this bioerosion may be intensified when their natural predators 
(some fish and molluscs) are overfished. 

Tourism may strengthen the economy of reef areas, but it often has the effect 
of damaging or destroying that very resource which people pay to see. 
Tourism leads to increased development, which may increase erosión and 
siltation over the reef, and it almost certainly increases the amount of 
sewage entering the water. As more tourists arrive, more fish is sold to 
restaurants and hotels, and more shells and coral are removed as mementos. 
Even seemingly innocent activities can have repercussions on the reef 
ecology. Reefs off Florida in the United States have been seriously damaged 
by amateur boaters colliding with submerged coral heads or anchoring to 
coráis which are easily broken in the process. Even walking on reefs at low 
tide is destructive to coral. Tropical countries that wish to have long-term 
economic benefits from their local reefs would do well to avoid 
overexploitation, and to edúcate both local populations and visiting tourists. 

In the 1970s, attention was drawn to Pacific reefs that were being destroyed 
by Acanthaster planci, the crown-of-thorns starfish. This large (30-40 cm 
diameter) echinoderm feeds on coral polyps, but is normally present in low 
enough numbers that reef damage is slight. However, Acanthaster began to 
undergo population explosions on many reefs in the western Pacific, 
including the Great Barrier Reef. Tens of thousands of starfish were found 
on some reefs, and the impact of so many large predators was devastating, 
with entire reefs being destroyed in some areas. For example, in less than 
three years, Acanthaster destroyed approximately 90% of the coral along 
38 km of reef off Guam. Parts of the Great Barrier Reef have experienced 
major damage from predation during two sepárate starfish outbreaks, one in 
the 1960s and again in the 1980s. 

Many causes have been advanced to explain the outbreaks of Acanthaster . 
These inelude increases in waterborne pollutants or increases in 
sedimentation due to dredging or other activities. It also was suggested that 
shell collecting was to blame. Many tritons had been removed from reefs, as 
the shell of this large snail is a prized ornament. Tritons are one of the few 
natural predators of the crown-of-thorns starfish, and the decline of these 
snails could result in lessened mortality of Acanthaster. It seems, however, 
that no one explanation applies to all the reefs damaged by starfish 
predation. There have been suggestions that Acanthaster density fluctuations 
may be natural eyeles that are linked to ocean climate change. As with the 
similar kelp-urchin interactions discussed in Section 8.3, the debate 
continúes about whether starfish population explosions are a contemporary 
phenomenon linked with human activities, or whether they are an ecological 
pattern that has persisted for thousands of years. In any case, restoration of 
coral cover takes place within 10 to 20 years, but it may take much longer to 
re-establish the original species diversity. 

Storms, exposure during exceptionally low tides, and other natural events 
may cause widespread coral damage. In 1982-83, a rapid 2-4 C° rise in 
seawater temperature to nearly 30°C was caused by a particularly strong El 
Niño event, and this damaged or killed 95% of the coral in the Galápagos 


Islands, and 70% to 90% of the coráis in the Gulf of Panama and in 
Indonesia. In 1987, reefs throughout the Caribbean were affected by a 
similar event. Coráis are particularly sensitive to elevated temperatures as 
they live so cióse to their upper temperature tolerance limits. Any sustained 
temperature increase usually results in bleaching due to loss of zooxanthellae, 
and eventually to death if the thermal stress continúes. In Hawaii, bleaching 
of coral has been correlated with discharge of heated effluent from a power 
plant, and bleaching can be experimentally induced by elevated temperature. 
On a world-wide basis, global warming poses a serious threat to coral reefs. 

Reef-building coráis similar to modern species have a geological record 
dating back about 250 million years, but other types of coral reefs occurred 
as long ago as 500 million years. More than 5000 species of extinct coráis 
are known, compared with the present number of less than 600 reef-building 
species. The pace of species extinction may be accelerating in many areas. 
Some countries, recognizing the benefit of adjacent reefs, have developed 
governmental policies to protect them. About 65 countries now have almost 
300 protected areas that inelude coral reefs; these inelude marine reserves 
and underwater parks. The economic benefits of controlled tourism may push 
other countries to develop conservation policies. 


1 The annual fish harvest of more than 100 x 10 6 tonnes has had a greater 
impact on the ocean than any other human activity. By 1995, ñsh catches 
were declining in all major oceans except the Indian Ocean and 70% of the 
ocean’s fish stocks were either being fully exploited, were overfished, or 
were recovering from being overfished. 

2 The results of intensive fishing inelude: declines in targeted fish stocks 
and consequent changes in relative abundance of species; changes in size 
structure of fish populations; declines in pelagic and benthic animáis 
captured incidentally as by-catch; acceleration of nutrient transfer to deep 
water through dumping of dead by-catch; increased numbers of scavengers 
in marine food chains receiving large amounts of by-catch; and destruction 
of seabed habitats through benthic trawling. 

3 Fish stocks are also affected by construction of dams that eliminate 
spawning grounds (e.g. salmonids), and by loss of coastal spawning and 
nursery grounds due to land reclamation or pollutiott. 

4 Human activities result in the release of a variety of poliutants into the 
sea. These substances, which may cause deleterious changes, inelude 
Petroleum hydrocarbons, plastics, pesticides and related chlorinated 
hydrocarbons, metáis, fertilizers, and radioactive wastes. Sewage outfalls 
deliver many of these poliutants as well as human wastes, detergents, and 
pathogenic bacteria and viruses. Power plants also release heated effluents 
that elevate ambient seawater temperatures. 

5 Although oil spills are among the most visible types of marine pollution 
and ecological damage in the immediate site may be severe, populations 
generally recover within 5 to 10 years. In the open ocean, the accumulated 
concentration of petroleum hydrocarbons is too low to cause measurable 


6 Nonbiodegradable plástic materials are now found throughout the oceans. 
These inelude lost fishing nets, which may continué to entangle animáis for 
years, and plástic materials that are mistaken for prey and ingested by turtles 
and seabirds. 

7 Toxic synthetic pesticides like DDT and related compounds (dioxins, 
PCBs) that enter the marine environment are not readily degraded; they 
persist for long periods and enter marine food chains. Because they are 
stored in fat tissues, these compounds show biomagnification, with higher 
trophic level animáis accumulating concentrations that may be lethal. Past 
incidences of pesticide and PCB poisoning in marine organisms have led to 
bans on usage and production in some countries. 

8 Accumulations of heavy metáis (e.g. mercury, copper) resulting from 
industrial outfalls may cause serious human health problems. Historically, 
the damaging effeets caused by humans eating mercury-contaminated 
seafood were shown in the 1950s in Minamata, Japan, where more than 
2000 people were directly affected. Now regulations and monitoring 
programmes exist to limit and detect unacceptable concentrations of these 
metáis in marine produets. 

9 Sewage disposal is a major form of Coastal pollution throughout the 
world. Nutrients in human wastes and those in detergents and fertilizers 
enrich local waters. This eutrophication may be beneficial in some cases, but 
often the amount of nutrients delivered leads to excessive plankton blooms 
that eventually decay and cause oxygen depletion. Pathogenic organisms in 
human wastes, like the cholera virus, can be filtered out of water near 
sewage outfalls by mussels and clams, and then be transmitted to humans 
who consume this seafood. 

10 Radioactive wastes do not presently occur in concentrations that 
threaten marine Ufe, although it is known that some organisms, particularly 
seaweeds and bivalves, can accumulate radionuclides from waters around 
nuclear plants. 

11 Power plants release heated water that elevates ambient seawater 
temperature and thereby affeets marine communities within the immediate 
area. In some cases the heated effluent is used to enhance growth rates of 
organisms grown in culture, but often the community changes are 
detrimental or unwanted. 

12 Some marine communities and ecosystems have been changed through 
the delibérate or accidental transplantation of species. Increased commercial 
shipping has accelerated the rate of introduction of species into new 
environments, with an estimated 3000 species of marine plants and animáis 
being carried daily across oceans in the ballast tanks of ships. Many do not 
survive but some, like the ctenophore Mnemiopsis or the zebra mussel, have 
major impaets on their new environments. 

13 Estuaries and mangrove swamps are productive Coastal ecosystems that 
constitute important spawning and nursery grounds for many fish, harbour 
shellfish populations, and provide rich feeding grounds for birds. As well, 
mangrove swamps buffer coastlines from erosión and inundation during 
tropical storms. However, these ecosystems are often heavily affected by 
human activities such as land reclamation, disposal of sewage and industrial 
wastes, and eutrophication. 


14 Coral reefs are declining throughout the world. Expanding human 
populations near reefs and increasing tourism have accelerated development 
and have brought growing pressure to exploit reef resources. Coral reefs are 
detrimentally affected by increased sedimentation resulting from land 
development and subsequent erosión, and from eutrophication stemming 
from sewage disposal and agricultural runoff. In some locales, the coral is 
mined as building material. Destructive fishing techniques remove large 
numbers of fish, change species composition on the reef, and damage coráis 
directly. Coráis world-wide have been affected by elevations in seawater 
temperature, and global warming is a potential danger to reef communities. 

Now try the following questions to consolídate your understanding of this 

QUESTION 9.5 The subject of human impacts on marine bioia is much too 
large to cover fully in this chapter. What other impacts can you ihink of that 
might have been included? 

QUESTION 9.6 Is ti possi ble to harvest fish from the sea without causing 
changes in marine ecosystems? 

QUESTION 9.7 Does sewage disposal al sea have any beneficia! impact? 

QUESTION 9.8 Estuary A rece i ves freshwaier with a htgh sediment load, 
whereas Estuary B receives retalively clear river water. Both receive 
approximately equal amounts of urban pollutants. Which estuar> r would have 
lower concentrations of these pollutants in the water, and which would have 
lower polluiani concentrations in the sediments? 



Approx. time unit began 

(millions of years ago) 




Major biotic events 




Modern man appears 



Early humans appear (2-3 mya) 









Early anthropoids appear (40 mya) 
Heteropods appear 



Marine mammals, shelled pteropods appear 
(55 mya) 

Seabirds appear (60 mya) 



Dinosaurs become extinct (65 mya) 

Diatoms and silicoflagellates appear 

(100 mya) 



Ammonoid cephalopods become extinct 
Coccolithophorids and forams appear; 

number of dinoflagellate species 





Marine reptiles and primitive birds appear 
(200 mya) 

Most nautiloid cephalopods go extinct 



Dinosaurs and mammals appear; most 

nautiloid cephalopods become extinct 



Trilobites become extinct 

Teleost fishes appear (300 mya) 






Land plants appear (420 mya) 



Dinoflagellates, barnacles appear; 

bivalves become abundant 
Elasmobranchs (sharks), ammonoid 

cephalopods appear (400-450 mya) 



Tintinnids evolve, coral reefs form 
Primitive fish appear (550 mya) 



Trilobites dominate; ostracods, echino- 

derms, nautiloid cephalopods appear 
Marine algae become highly diversified 


Radiolaria appear (600 mya) 

Abundant fossils of marine invertebrates 

(e.g. jellyfish, sponges, molluscs) 
Oldest fossils of shelled marine animáis 

(650 mya) 

First multicellular seaweeds (800 mya) 


Calcareous algae (1500 mya) 


Multicellular life appears (2100-1900 mya) 
Earliest photosynthetic organisms 

(2800-2500 mya) 


Oldest fossils of cyanobacteria (3500 mya) 
Life originates (3900-3500 mya) 


Atmosphere and ocean form (4400 mya) 


Earth’s crust forms (4600 mya) 



1 squarecentimetre(cm 2 ) 

= 100 mm 2 = 0.155 square inch 

1 square metre (m 2 ) 

= 10 4 cm 2 =10.8 square feet 

1 square kilometre (km 2 ) 

= 10 6 m 2 = 247.1 acres 

1 hectare (ha) 

= 10000 m 2 


molar concentration (M) 

= gramme molecular weight per litre 

¡ig litre -1 

mg m -3 

parts per million (ppm) 

= mg litre -1 

parts per billion (ppb) 

= /xg litre -1 

parts per trillion (ppt) 

= 10 -3 /xg litre -1 = 1 nanogramme litre - 

/xg litre 1 -r molecular weight = /xM = /xmol litre 


1 ángstrom (Á) 

= 0.0001 micron 

1 nanometre (nm) 

= 10 -9 metres 

1 micron (/x) 

= 0.001 millimetre (or 10 -3 mm) = 10 -( 

1 millimetre (mm) 

= 1000 microns = 0.001 metre 

1 centimetre (cm) 

= 10 millimetres = 0.394 inch 

1 decimetre (dm) 

= 0.1 metre 

1 metre (m) 

= 100 centimetres = 3.28 feet 

1 kilometre (km) 

= 1000 metres = 3280 feet 

1 kilometre (km) 

= 0.62 statute mile = 0.54 nautical mile 

1 inch (in) 

= 2.54 centimetres 

1 foot (ft) 

= 0.3048 metre 

1 yard (yd) 

= 3 feet = 0.91 metre 

1 fathom 

= 6 feet =1.83 metres 

1 statute mile 

= 1.6 kilometres = 0.87 nautical mile 

1 nautical mile 

= 1.85 kilometres =1.15 statute miles 


1 milligram (mg) 

= 0.001 gramme 

1 kilogram (kg) 

= 1000 grammes = 2.2 pounds 

1 tonne (t) 

= 1 metric ton = 10 6 grammes 

1 pound (Ib) 

— 453.6 grammes 


1 minute (min) 

= 60 seconds (s) 

1 hour (h) 

= 3600 s 

1 day (d) 

= 86400 s 

1 year (yr) 

= 365 d 



1 kilometre per hour = 27.8 centimetres per second 
1 knot (kn) = 1 nautical mile per hour = 51.5 centimetres per second 


1 millilitre (mi) = 0.001 litre = 1 cm 3 (or 1 cc) 

1 litre (1) = 1000 cm 3 = 10" 3 m 

1 cubic metre (m 3 ) = 1000 litres 


1 einstein (E) = 6.02 x 10 23 photons = 1 mole of photons 
1 watt m" 2 ^ 4.16 ± 0.42 ¡i einsteins m -2 s _1 
(The above relationship applies only to PAR) 

1 joule m~ 2 s _1 = 1 watt m -2 
1 calorie cm -2 min _l ~ 700 watts irf 2 
1 langley = 1 calorie cm -2 


1 calorie (cal) = 4.184 joules (J) 

1 kilocalorie (kcal) = 1000 cal 
1 gramme carbón (g C) ^ 10 kcal 

1 gC^2 g ash-free dry weight (where ash-free dry weight is dry weight 
less the weight of inorganic components such as shells) 

1 g ash-free dry weight ^ 21 kJ 
1 g organic C «s 42 kJ 
1 g ash-free dry weight ^ 5 g wet weight 
1 litre O 2 = 4.825 kcal 
1 g carbohydrate ^ 4.1 kcal 
1 g protein ^5.65 kcal 
1 g fat ^ 9.45 kcal 


CUSHING, D. H. (1975) Marine Ecology and Fisheries , Cambridge 
University. This book begins with a review of marine production cycles, 
which are then related to the biology and population dynamics of 
commercial fish stocks; it concludes with a discussion of fluctuations in fish 
stocks caused by natural events and by human exploitation. 

DüXBURY, A. C. and DUXBURY, A. (1994) An Introduction to the World's 
Oceans , (4th edition), Wm. C. Brown. A general, easy to read overview of 
the oceans, including introductory material on physical, geological, Chemical 
and biological oceanography. 

FRASER, J. (1962) Nature Adrift, the Story of Marine Plankton , G. T. Foulis. 
A well illustrated and informative account of planktonic organisms written in 
an easily understandable manner. 

Gage, J. D. and Tyler, P. A. (1991) Deep-sea Biology: A Natural History 
of Organisms at the Deep-sea Floor , Cambridge University. A recent review 
of deep-sea biology, including information on hydrothermal vent 

Hardy, A. (1970) The Open Sea: Its Natural History . Parí 1: The World of 
Plankton , (2nd edition), Collins. A classic account of plankton, delightfully 
written and illustrated with watercolour drawings done by the author while 
at sea. / 

Laws, E. A. (1993) Aquatic Pollution: An Introductory Text , John Wiley. A 
thorough introduction to the sources and consequences of anthropogenic 
pollution in the sea and in freshwater. 

Mann, K. H. and LAZIER, J. R. N. (1991) Dynamics of Marine Ecosystems, 
Biological-Physical Interactions in the Oceans , Blackwell. A comprehensive 
treatment of the links between water circulation patterns and biological 
processes; although the physical oceanography is at a fairly elemental level, 
some mathematical knowledge is necessary. 

MARSHALL, N. B. (1980) Deep Sea Biology: Developments and Perspectives , 
Garland STPM Press. A descriptive account of deep-sea invertebrates and 
fish including morphological, behavioural and physiological adaptations. 
McClüSKY, D. S. (1989) The Estuarine Ecosystem , (2nd edition), Blackie. 
An introduction to estuaries, emphasizing biological aspects and including a 
discussion on pollution and management. 

NYBAKKEN, J. W. (1988) Marine Biology: An Ecological Approach , (2nd 
edition), Harper & Row. A well written, well organized treatment of marine 
biology, particularly recommended for its discussions of benthic 

PARSONS, T. R., Takahashi, M., and HARGRAVE, B. (1984) Biological 
Oceanographic Processes, (3rd edition), Pergamon. A more advanced 
treatment of biological oceanography that emphasizes production processes; 
minimal mathematics. 

RAYMONT, J. E. G. (1980) Plankton and Productivity in the Oceans , (2nd 
edition). Vol. 1, Phytoplankton\ Vol. 2, Zooplankton , Macmillan. Classic, 
comprehensive reviews of plankton including descriptions, biology, 
distribution patterns, and abundance. 



QUESTION 1.1 The overlap occurs because plankton and nekton are separated 
strictly according to swimming ability, whereas the classification scheme in 
Figure 1.2 is based on size as well. Therefore, the figure shows that very 
large zooplankton with feeble swimming ability (e.g. some jellyfish) may be 
the same size, or even larger, than some nekton (e.g. fish). 

QUESTION 1.2 Biological oceanography was slow to develop largely because 
of the inaccessibility of many areas of the oceans. Global systematic 
observations and collections at sea in all depths require large ships operating 
for relatively long time periods, as well as specially designed gear. These 
requirements make sea-going operations very expensive compared with 
land-based research. 

QUESTION 1.3 Primitive fish first appeared in the seas about 550 million 
years ago. 

QUESTION 1.4 At 3000 m depth, there is no light and consequently no plant 
life. The water temperature is low (2-4°C) and constant. Hydrostatic 
pressure is relatively high. Relative to conditions near the sea surface, 
nutrients like nitrate and phosphate are present in higher concentrations, but 
food supplies for animáis are less abundant. 

QUESTION 1.5 In 1960 the bathyscaphe Trieste carried two men to a depth of 
10916 m in the Mariana Trench in the Pacific Ocean. 


QUESTION 2.1 The solar radiation received at the surface of the Arabian Sea 
is: (a) about 3700 ¡jl E m“ 2 s' 1 in September, and (b) about 2900 (i E m~ 2 
s -1 in January. 

QUESTION 2.2 The extinction coefficient, k , can be determined from: 

\og e 100 - log e 50 

10 m 

= 0.07 m" 1 

QUESTION 2.3 This is due to the different penetration of various wavelengths 
in water. Red and yellow quickly disappear with depth, so that these colours 
can no longer be seen at diving depths. Green and blue wavelengths 
penétrate deeper and are still visible at the depths of reefs. Using a flash on 
underwater cameras restores the entire colour spectrum and the true colours 
of the reef are seen on the film. 

QUESTION 2.4 Moonlight is obviously too weak to cause photosynthesis. 
However, notice that moonlight is sufficiently intense (even down to 600 m 
in clear oceanic water) to be seen by deep-sea fishes. Since they often 
respond to light by carrying out vertical migrations, the intensity of 
moonlight could have an effect on their movements in the water column. 
Moonlight could also facilitate detection of prey by predators that use visión. 

QUESTION 2.5 The higher the salí contení of the water, the lower the 
submerged weight of the organism. Consequently, organisms expend less 
energy to avoid sinking in water of higher salinity. 


QUESTION 2.6 Both types of water (a and b) lie on the density contour 
described by a sigma-í valué of 26.0. Using the equation given in the figure 
caption, 26.0 = (d — 1) x 1000, or d = 1.026 g cm -3 . The density of 
seawater described in (a) and in (b) is the same. 

QUESTION 2.7 The initial freezing point will be slightly higher than that of 
average seawater with a salinity of 35, and freezing will start sooner. 
However, sea-ice formation itself results in increasing the salt content of the 
surrounding water, and this depresses the freezing point, thus inhibiting 
further ice formation. 

QUESTION 2.8 This is known as the Antarctic Circumpolar Current, or West 
Wind Drift, that flows around the continent of Antárctica. 

QUESTION 2.9 From Appendix 2, approximately 4.16 ¡i einsteins m -2 s -1 
equals 1 watt m~ 2 . Therefore, 1 ¡jl E m -2 s -1 is about 1/4 or 0.25 W m -2 , 
and 10 ¡i E m -2 s~ ] is about 10/4 or 2.5 W irr 2 . 

QUESTION 2.10 This relates to the small temperature range of the sea 
compared with land. Many marine organisms, and residents of deep water in 
particular, experience only relatively small fluctuations in ambient 
temperature. Homoiothermic terrestrial animáis, which are able to regúlate 
internal body temperature, are better adapted to the wider environmental 
temperature range encountered on land. 

QUESTION 2.11 (a) This is caused by an excess of precipitation over 

evaporation in the rainy belt around the Equator. 

(b) The Arctic basin receives large amounts of freshwater from major 
rivers in Cañada and Siberia. 

QUESTION 2.12 A combination of low temperature and high salinity results 
in very dense water. 

QUESTION 2.13 This is in the aphotic zone where there is no light. The water 
temperature will be 4°C or below, and the salinity will be about 35. This 
coid, high salinity water has a high density (from Figure 2.14, a t ~ 27.75). 
Hydrostatic pressures will exceed 200 atm. 


QUESTION 3.1 The volume of a sphere is calculated from: 4/3^r 3 . Cancelling 
out the expression 4/37T, which is the same for both species, and converting 
diameter to radius by dividing by 0.5, gives r 3 valúes of (0.5) 3 for 
Synechococcus cell volume and (25) 3 for the dinoflagellate. Dividing (25) 3 
by (0.5) 3 gives 125 000 Synechococcus cells, the number needed to produce 
the equivalent volume of one 50 /xm-diameter dinoflagellate. So, although 
the concentrations of Synechococcus in the sea may be very high in terms of 
numbers per millilitre, their actual biomass (numbers x volume) may be 
quite low relative to other phytoplankton. 

QUESTION 3.2 Since different wavelengths of light ha ve different extinction 
coefficients and penétrate to different depths, algae with different accessory 
pigments can take advantage of trapping wavelengths that are not captured 
by chlorophyll a and may thus be able to extend their vertical range, or live 
at depths not inhabited by other photosynthetic species. 


QUESTION 3.3 Calculating P g for each species, 

Species 1 : P g = = 1.6 mg C mg” 1 Chl a h” 1 

6 x 50 i 

Species 2 : P 2 =-— 4.3 mg C mg Chl a h 1 

K *20 + 50 6 6 

Therefore species 2 would be growing faster at the specified light intensity. 

QUESTION 3.4 Calculating the critical depth from equation 3.6, 

D a 

500 x 0.5 
0.07 x~10 

= 357 m 

Because the critical depth (357 m) is greater than the depth of mixing 
(100 m), there is net photosynthesis in the water coiumn. 

QUESTION 3.5 (a) Obviously more carbón is being fixed photosynthetically 

in area B (50 mg C m“ 3 h” 1 ) than in area A (20 mg C m” 3 h” 1 ). However, 
this is not a comparative measure of photosynthetic activity. Instead, the 
assimilation index can be used to compare areas A and B. This índex is 
expressed as the amount of carbón fixed per quantity of chlorophyll a per 
hour, so for area A this would be 20 mg C 2 mg' 1 Chl a h” 1 , or 
10 mg C mg” 1 Chl ah -1 . The valué of the assimilation index for area B is 
only 2 mg C mg” 1 Chl ah” 1 , indicating lower photosynthetic activity of the 

(b) The difference in photosynthetic activity could be due, for example, to 
the phytoplankton in area B being at the end of a bloom, and those in area A 
growing in conditions of high nutrient concentrations at the beginning of a 

QUESTION 3.6 The K N valúes for nitrogen uptake are considerably higher in 
eutrophic waters compared with oligotrophic regions. Phytoplankton species 
living in oligotrophic waters can take up nitrate (or ammonium) at ambient 
concentrations of < 0.1 fx M. In contrast, phytoplankton in eutrophic waters 
generally need nitrate concentrations of > 1.0 ¡jl M. 

QUESTION 3.7 The stratification index is calculated from equation 3.13 as: 

i 5000 cm 

108,0 0.003 x (3.3) 3 

= 5.5 

Frontal zones are usually formed when S ^ 1.5. The calculation indicates 
that no frontal zone will occur as a result of tidal fiow over this bank. 

QUESTION 3.8 Although much of the Indian Ocean lies between latitudes 
that receive high amounts of solar radiation, the basic circulation pattern is 
anticyclonic (see Figure 2.19). This results in convergence of surface water 
toward the central area of the gyre and deepening of the thermocline. 
Consequently, nutrient levels in the euphotic zone are comparatively low and 
primary production is nutrient-limited. 

QUESTION 3.9 The many species of marine phytoplankton differ in their 
requirements for light and essential nutrients. They also contain different 
photosynthetically active pigments in different relative amounts and therefore 
can absorb different wavelengths. Certain species are shade-adapted and so 
live at deeper depths or under ice. Others are better able to carry out 


photosynthesis at lower (or higher) temperatures. Some can survive in 
environments where nutrient concentrations are relatively low. All of these 
differences may sepárate species spatially (in depth or geographically), or 
temporally as conditions change and favour one species over another. Thus 
what appears superficially to be a homogeneous environment is actually one 
that contains microenvironments of differing and variable light intensities 
and nutrient concentrations. 

QUESTION 3.10 In equation 3.7, 

(X 0 + AX) = V' 

and therefore, 

\og e (X 0 + AX) - log e (X 0 ) = ¡it 


_ log^Xo + AX) - log^Xo) 

Substituting a valué of 2.5 mg C m -3 for Xo, and AX being equal to 
0.2 mg C m~ 3 in 1 hour gives: 

log,(2.5 + 0.2)-log,2.5 


fi = 0.9933 - 0.9163 h" 1 = 0.077 Ir 1 
Using equation 3.10, 

d = 0.69/0.077 h' 1 = 8.9 h 

QUESTION 3.11 (a) A doubling time of about 9 hours translates to 

approximately 2.7 generations per day (from 24 h/9 h), and this is a rapid 
rate of growth. 

(b) This growth rate is typically found in phytoplankton living in tropical 
upwelling regions. 

Species B 
(2 doublings / day) 

Question 3.12 

QUESTION 3.12 With the aid of graph paper, you can show that the growth 
rate of species A will already have reached 1 doubling per day at 0.2 ¡i M 
nitrate. The growth rate of species B will still be only about 80% that of 
species A at 0.4 \i M nitrate, and therefore species A will dominate at this 

QUESTION 3.13 Different species of phytoplankton generally will be growing 
below their ¿¿ max growth rates. This is because each will be limited by 
different K^ valúes for different nutrients. Therefore, the K # valúes are more 
important than the different /x max valúes in determining species diversity. 

QUESTION 3.14 Theoretically yes, and this can be demonstrated 
experimentally. However, the logistics of applying and maintaining a large 
concentration of nutrient media over wide areas is generally economically 
prohibitive. In addition, this would not be realistic in the sea because there 
are so many different phytoplankton species with different requirements that 
it would probably be impossible to selectively enhance one particular type. 
As well, there are so many other variables (e.g. predation) controlling 
phytoplankton species composition in the sea that the results of adding a few 
nutrients would probably not favour a selected species. On the other hand, 


nutrient addition in relatively small restricted areas could increase total 

QUESTION 3.15 At this high latitude, there will be phytoplankton production 
only during those months when there is sufficient solar radiation to support 
photosynthesis. Ice is relatively transparent to light, but a thick ice cover will 
cause a reduction in the amount of PAR light available to phytoplankton, and 
the dominant species will be those that are adapted to live in low light levels 
(i.e. shade-adapted). Water temperatures will be very low (< — 1°C), and this 
reduces the activities of plants (see Section 2.2). There are changes in 
salinity when sea-ice forms (see Section 2.4), and the gradient in salinity 
between the ice and surrounding water may affect species composition of the 
associated phytoplankton. In spite of the rigorous environmental conditions, 
certain phytoplankton are able to grow immediately under the ice and also 
within the ice fractures. They form an important part of the food chain in 
polar regions. 

QUESTION 3.16 There are at least four ways in which this can happen, one of 
them general, the others more specific. 

1. All types of phytoplankton may produce very large blooms that 
eventually decay, causing lowered oxygen concentrations and death of 
animáis that cannot avoid these anoxic areas. 

2. A few dozen species of dinoflagellates produce saxitoxin which is 
transferred via the food chain to vertébrate animáis that are sensitive to this 
neurotoxin. Fish, birds, and marine mammals may suffer poisoning or death 
from the accumulation of saxitoxin, and humans develop paralytic shellfish 
poisoning from consuming shellfish that have fed on toxic dinoflagellates. 

3. One diatom (. Pseudonitzschia ) produces a neurotoxin called domoic acid 
that has similar effects on marine animáis and on humans who eat 
contaminated shellfish. 

4. Ciguatera fish poisoning, which originates with a toxic dinoflagellate, is 
a common health problem in many tropical and subtropical countries. 


QUESTION 4.1 The pelagic environment lacks hiding places for animáis 
seeking refuge from predators. Transparency permits background colours to 
be transmitted through an animaks tissues, and thus provides an ideal way of 
escaping detection by visual predators. 

QUESTION 4.2 Asexual budding allows salps to respond to favourable 
environmental conditions by rapidly producing large numbers of new 
individuáis, and it is therefore common to find salps in large swarms. 
However, all individuáis produced by budding of one solitary form are 
genetically identical. The establishment of swarms of salp chains, in which 
many non-identical sexual individuáis are in cióse proximity, favours 
cross-fertilization, and this process restores genetic variability in the 

QUESTION 4.3 Red wavelengths of light are quickly absorbed and scattered 
in near-surface water. The only light present in deeper water is in the 
blue-green part of the spectrum. Red coloured bodies are difficult or 
impossible to see in blue light as the red is not reflected. 


QUESTION 4.4 Most individuáis migrate toward the surface at night, but the 
adults have a deeper daytime distribution than their young. 

QUESTION 4.5 These copepods move into shallower depths in spring and 
early summer to take advantage of abundant food during the period of high 
primary productivity. In winter, when food is no longer abundant at the 
surface, these copepods move deeper where they subsist on stored fat or, in 
the case of C. helgolandicus, they may also feed carnivorously on other 
zooplankton. By living in deep water during winter, copepods avoid surface 
turbulence caused by storms, and their mortality due to predation may be 
reduced by remaining in dark waters. 

QUESTION 4.6 (a) The density of phytoplankton (as measured by 

chlorophyll a) is often inversely correlated with zooplankton numbers due to 
grazing by herbivores. However, the change in chlorophyll could also result 
from differences in nutrient concentration in the surface water over the 
80-km distance that was sampled. 

(b) Zooplankton samples were taken at night-time from a shallow depth 
(3 m). Because of diel vertical migration, it would be expected that the 
numbers of zooplankton would decrease significantly at this depth during 
daylight hours. 

QUESTION 4.7 Crustaceans have rigid exoskeletons. In order to increase in 
size and add more appendages, they musí first shed the exoskeleton, expand 
the body, and then make a larger exoskeleton. 

QUESTION 4.8 Most benthic invertebrates are slow-moving with limited 
mobility or are permanently attached to a substrate, and they remain in 
restricted regions throughout their adult lives. The production of 
meroplanktonic larvae, which are carried in ocean currents, ensures wider 
distribution for a species than could be attained by the benthic adults. 

QUESTION 4.9 Among the phytoplankton, diatoms and silicoflagellates 
produce siliceous skeletons and coccolithophorids form calcareous plates, all 
of which can be found in sediments. Among the zooplankton, calcareous 
tests or shells are formed by foraminifera, heteropods, thecosomes, larval 
gymnosomes, and veliger larvae of benthic molluscs; and the radiolarians 
form siliceous skeletons. The skeletal remains of these animáis are present in 
sediments, with those of forams, radiolarians and thecosomes being most 

QUESTION 4.10 Carnivorous groups inelude the Cnidaria, Annelida, 
Chaetognatha, Amphipoda, and Clione limacina (a gymnosome). The 
zooplankton that are predominantly herbivorous inelude the euphausiids, 
salps, most of the copepods, and both species of Limacina (thecosomatous 

QUESTION 4.11 If plankton were evenly dispersed, it would be in such dilute 
concentrations that it would be difficult for grazers and predators to obtain 
enough food. When food occurs in patches, animáis can expend less energy 
to obtain much more food, and this enables them to grow faster and thus 
enhances their own survival. This explanation only applies, however, to 
those organisms that are capable of actively locating high concentrations of 
food. Some planktonic animáis do not necessarily rely on patches of prey 
and instead utilize passive foraging methods. For example, many medusae, 


siphonophores, and ctenophores have tentacles that can be extended for 
considerable distances into the surrounding water to capture more 
dispersed prey. 

QUESTION 4.12 Neocalanus cristatus undergoes a seasonal migration that is 
associated with reproduction and development. Figure 4.22 shows the 
epipelagic summertime distribution of the copepodite V stage, which will 
begin to migrate deeper at the end of the summer before maturing into stage 
VI (the adult). The adults overwinter and lay eggs in deep water, at depths 
between 500 and 2000 m; young stages of copepodites migrate upward as 
they develop. 


QUESTION 5.1 (a) Total numbers of individual organisms will decrease in 

succeeding trophic levels; for example, there are many more herbivorous 
zooplankton than there are fish and top-level predators. 

(b) There is considerably more total primary production than secondary 

QUESTION 5.2 E T would equal 0.166 (from 25 g C m -2 yr~* divided by 
150 g C m 2 yr _1 ). Expressed as a percentage, about 17% of the net primary 
production is being transferred to the production of herbivorous copepods. 

QUESTION 5.3 In general, as the numbers of prey diminish in higher levels, 
the predators become more active and spend more energy in seeking food. 
Respiration losses become relatively higher in more active animáis. 
Consequently, the production of higher trophic level species is lower relative 
to production in preceding levels, and E T valúes are lower. 

QUESTION 5.4 As less of the primary production is transferred to the next 
trophic level in the pelagic food chain, the valué of Ej will decrease. 

QUESTION 5.5 (a) The biomass in all trophic levels is approximately one 

order of magnitude higher in the Antarctic Ocean. 

(b) The surface waters of the Antarctic Divergence are upwelled and 
therefore high in nutrients, and this leads to high primary productivity; the 
dominant producers are chain-forming diatoms that are consumed directly by 
large zooplankton (euphausiids), and these animáis are the major food of 
baleen whales; and, because the food chain is short, comparatively less 
energy is lost between primary producers and the highest trophic level. 

QUESTION 5.6 Employing equation 5.2 and setting n = 2, the máximum 
amount of herring which could be produced by this food chain would be: 

P(„ +l ) = 300 g C m -2 yr" 1 x (0.1) 2 = 3 g C rn^yr” 1 . 

QUESTION 5.7 A number of other planktonic protozoans are bactivorous; 
these inelude foraminiferans, radiolarians, and ciliates (especially tintinnids). 
In addition, salps and a number of different types of meroplanktonic 
invertebrate larvae can capture and consume bacteria. 

QUESTION 5.8 The North Pacific gyre is a región of low primary and 
secondary productivity because of low nutrient concentrations (see 
Section 3.5.1); consequently, there are few nutritive substrates available for 


bacterial growth. Freshwater localities receive both nutrients and organic 
materials from runoff. Freshwater primary production is high (as shown in 
Figure 5.8), and bacteria can utilize organic materials derived from the 
phytoplankton as well as from runoff. 

QUESTION 5.9 From equation 5.4, 

0.15 + 0.6 

P t = (80 - 30)---+ (30 x 0.6 - 80 x 0.15), 

= 50(0.375)+ (18- 12), 

= 18.75 + 6, 

= 24.75 mg m" 2 , 

Therefore, average production per day was 0.56 mg m“ 2 day" 1 , or about 2% 
day" 1 . 

QUESTION 5.10 (a) Using equation 5.10 to solve for K\, 

= 5 -° _ mg x 100% = 67 % 

7.5 mg 

(b) Using equation 5.11 to solve for Ki, 

5.0 mg 

Kj = 

(7.5 mg)(0.9) 

x 100% = 74% 

QUESTION 5.11 In general, large diatom chains are consumed by relatively 
large zooplankton such as euphausiids. On the other hand, small flagellates 
are generally consumed by protozoans. Thus a shift in the size of the 
primary producer would probably cause a marked change in the type of 
dominant herbivores, with subsequent changes at all higher trophic levels. 
The length of the food chain would be expected to increase, with more 
trophic levels being added to the System in which flagellates are the major 
primary producers. 

QUESTION 5.12 (a) The forcing functions are the amount of light available 

and the nutrient concentrations. 

(b) Physiological functions inelude the reaction of light with 
phytoplankton; grazing by zooplankton and zooflagellates; predation by 
ctenophores, salmón, and microzooplankton; bacterial decomposition; and 
the growth rates of all organisms. Note that phasing functions, like 
temperature and light extinction coefficients, are not shown in this figure, but 
they are included in the actual model. 

QUESTION 5.13 A light extinction coefficient of 0.7 m" 1 indicates low light 
levels that severely slow the growth of phytoplankton. Thus zooplankton 
production is limited by the small amount of primary production. 

QUESTION 5.14 (a) If 30% of total production is regenerated, then we have: 

300 g C m" 2 yr" 1 x 0.3 = 90 g C m" 2 yr" 1 of regenerated production, and 

300 g C m" 2 yr -1 x 0.7 = 210 g C m“ 2 yr" 1 of new production. The 
/-ratio is calculated from new production divided by total production. 

/- ratio = 210/300 = 0.7. 


(b) This would be a región of moderately high production (see Table 3.5) 
where a relatively large amount of new nitrogen is entering the euphotic 
zone. A Coastal area, such as a continental shelf-break, could have such 

QUESTION 5.15 The exponential increase in sustainable fish yield is due to 
two factors, one being the increase in total primary production in eutrophic 
compared with oligotrophic conditions (as shown in Figure 5.21a). The 
second reason is because of the increased relative amount of new production 
to total production, as more new nitrate enters eutrophic waters from below 
the nutricline; this is shown in Figure 5.21b. 

QUESTION 5.16 Among the phytoplankton, the coccolithophorids build plates 
composed of calcite. Among the zooplankton, the foraminifera have calcitic 
tests and planktonic molluscs (heteropods, thecosomatous pteropods, and 
veliger larvae) usually produce shells of aragonite. The endoskeletons of fish 
and marine mammals also contain some CaCÜ 3 . In addition, very large 
amounts of calcium carbonate are present in coráis and the shells of benthic 

QUESTION 5.17 No. Carbón dioxide concentration is never limiting to 
photosynthesis because of the reservoir of CO 2 that can be released from 
bicarbonate and carbonate ions. However, nitrate concentrations may often 
be low enough to limit protein manufacture by phytoplankton and thus limit 
total primary production. 

QUESTION 5.18 Open ocean regions typically have six trophic levels, as 
shown in Figure 5.3. The answer can be derived from multiplying the 
transfer efficiencies in the successive trophic levels, as: 

1000 x 20/100 x (10/100 x 10/100 x 10/100 x 10/100) 

= 0.02 g wet wt m “ 2 yr _l 

QUESTION 5.19 In evolutionary terms, the short food chain (III in Figure 5.3) 
that culminates in baleen whales is the most recent; diatoms first appeared 
about 100 million years ago and whales about 55 mya. This food chain is 
also the most efficient in that it is the one that delivers the most energy from 
primary producers to terminal consumers. Parts of the open ocean food chain 
(I in Figure 5.3) are among the oldest in an evolutionary sense; green algae 
were present long before dinoflagellates and diatoms, and marine protozoans 
are known from 600 mya. However, 400 million years ago, the top predators 
of open ocean food chains would have been jellyfish, pelagic cephalopods 
(e.g. ammonites) or primitive fish (e.g. sharks), all of which were present 
before the appearance of bony fish (teleosts). 

QUESTION 5.20 The food chain of the región would have been shortened by 
one trophic level. If the average ecological efficiency were 10% between 
trophic levels, then theoretically the abundance of planktivorous fish 
available for harvesting should increase by an order of magnitude, if all other 
factors remain the same. However, in such circumstances, new predatory 
species often move into an area and replace those that were removed. 

QUESTION 5.21 Converting annual fish catch to grammes m 2 gives: 
0.5 tonnes x 10 6 g tonne -1 — 0.5 x 10 6 g wet wt/10000 m 2 . 


Converting wet weight of fish to dry weight gives: 

(0.5 x 10 6 g) x 0.2 = 10 5 g/10000 m 2 , or 10 g dry wt m -2 . 
Converting dry weight to carbón gives: 

10 g x 0.5 = 5 g C m _2 yr _1 . 

Employing equation 5.2, and setting n = 2 because there are two trophic 
transfers from phytoplankton to zooplankton to sand eels, the average 
ecological efficiency can be calculated from: 

5 g C m _2 yr _l = (200 g C m" 2 y r~ l )E 2 , 


E = ^5/200 = 0.16, orl6%. 

QUESTI0N 5.22 Case (a) might best be studied in an enclosed experimental 
ecosystem, where pesticides in varying concentrations could be added to 
enclosures containing large volumes of water and several trophic levels. No 
experimental container would be large enough to consider impacts of 
damming an estuary, so a Computer simulation model would be the best 
approach for Case (b). In Case (c), physiological properties of plants and 
animáis can be studied in the laboratory. 

QUESTI0N 5.23 Using equation 5.9 gives: 

A = $ ^ x 100% = x 100% = 85% efficiency 

5 mg 5 

QUESTI0N 5.24 Theoretically yes, but the danger lies in adding so much 
nutrient that it could cause an excessively large phytoplankton bloom. Most 
of this production could not be eaten by the resident oysters. Large amounts 
of phytoplankton would probably die and, as a result of the decomposition 
processes, a large oxygen demand could create anoxic conditions and the 
death of the oysters. Nutrient additions in such cases would have to be made 
with considerable caution and understanding of the system. 

QUESTION 5.25 The productivity of the area would eventually decrease 
because it would be robbed of the nitrogen that would have been regenerated 
from the fish community. 


QUESTION 6.1 Approximately 40 000 km (800 nets x 50 km length), 
extending from the surface to a depth of 8 or 10 m. Tied end to end, the 800 
driftnets would stretch around the world about 1^ times. 

QUESTION 6.2 New-born young can develop faster in warm water; in coid 
water, more energy would have to be used to maintain body temperature. 
Food concentrations, however, are generally much higher in summer months 
in cold-water environments than in tropical waters throughout the year, and 
both adults and juveniles benefit from rich feeding grounds. 

QUESTION 6.3 The major predators of Antarctic penguins are marine species, 
especially the leopard seal and toothed whales. There are no mammalian 


ground predators in the Antarctic, but one bird, the Antarctic skua, preys on 
the eggs and chicks of penguins. 

QUESTION 6.4 Their low fecundity makes them especially vulnerable to 
over-harvesting as they cannot rapidly replace their numbers through 

QUESTION 6.5 Both countries are situated along an intense continental 
upwelling zone that produces an almost continual supply of new nutrients to 
the euphotic zone. These nutrients are transferred through the food chain to 
produce larger numbers of fish. 

QUESTION 6.6 (a) Yes, any growth rate that is slower than that of the 

predators would theoretically result in 100% mortality due to predation. 

(b) In actuality, this would not happen for several reasons. As more and 
more prey are eaten and become scarcely distributed, it becomes 
energetically costly to seek them out, and predators generally switch to more 
abundant food. If the predators grow much more rapidly than the prey, they 
may tum to eating larger food sizes, thus obtaining more energy per item 

QUESTION 6.7 In general, fish grow most efficiently when they are small. 

The growth curve in Figure 6.11 indicates that older animáis (age 3+) do 
not increase greatly in size, but they do continué to eat prey Ítems, some of 
which are smaller fish. Therefore, the biomass yield will be highest and the 
fishery more efficient when large numbers of small fish are harvested rather 
than smaller numbers of large fish. For specific fisheries, however, the 
answer to this question may vary with the size at maturity, temperature, 
specific fishing methods, and the valué of the catch (e.g. salmón are larger, 
but more valuable than herring). Note that five out of eight of the world’s 
marine fisheries in Table 6.2 are based on species of small fish (anchovies, 
pilchards, capelin, and herring). 

QUESTION 6.8 The total marine catch is about 84 x 10 6 tonnes per year (see 
Section 6.7.1). Thus mariculture produces about 6% of this figure. 

QUESTION 6.9 Productive areas such as the leeward side of islands 
(Section 3.5.6), the mouth of estuaries (Section 3.5.5), and coid core rings 
(Section 3.5.1) should also support good fisheries because these are regions 
in which nutrient-rich subsurface water is mixed up into the euphotic zone. 

QUESTION 6.10 Theoretically, the lower down on the food chain that marine 
organisms are harvested, the much greater the potential harvest. There are, in 
fact, some zooplankton fisheries in the world (e.g. for Antarctic krill). 
However, it becomes more and more economically costly to harvest very 
small-sized organisms in the oceans. Therefore, unless special techniques are 
developed, it is probable that fish (including shellfish and squid) will remain 
the most easily harvested protein of the sea. 

QUESTION 6.11 The adult population size would change by 100% (99.90% of 
10 3 indicates that one fish will survive per spawning female; a mortality of 
99.95% indicates that only one fish per two spawning females will survive.) 
Notice that the change in mortality is statistically almost insignificant, but 
that the change in adult population numbers is 100%. These differences 


become even more dramatic when dealing with species, such as cod, that 
may lay more than 10 6 eggs per female per year. 

QUESTIQN 6.12 The removal of piscivorous fish from top trophic levels may 
increase the numbers of planktivorous fish and larger planktivorous 
invertebrates, including commercially undesirable species such as jellyfish. 
The targeting and removal of large numbers of one species may confer an 
ecological advantage to competing species which may then increase in 
number (review the consequences of removing large numbers of baleen 
whales from the Antarctic in Section 5.2). Harvesting much of the biomass 
from intermediate trophic levels may decrease the numbers of top-level 
predators such as tunas, toothed whales, and sharks. The incidental capture 
and dumping of large numbers of undesirable ‘trash’ fish may increase the 
number of pelagic and benthic scavengers. At present, none of these 
potential ecological consequences have been included in fisheries 
management theories but, in fact, commercial harvests do change the 
ecology of heavily fished regions. 

QUESTION 6.13 Because mariculture is carried out in Coastal regions, species 
should be tolerant to relatively wide temperature and salinity ranges. 

Because larvae require different conditions from the adults and are more 
sensitive to environmental change, it is easier to culture those species that 
have few life stages in their life cycle. Those species with tolerance to living 
and growing in crowded conditions are preferred by culturists, as are those 
with fast growth rates and ready marketability. 

QUESTION 6.14 From Table 5.1 (footnote), the total surface area of the ocean 
is about 362 x 10 6 km 2 . The whale sanctuary around Antárctica is 
28 x 10 6 km 2 , which represents about 8% of the world’s ocean. 


QUESTION 7.1 These categories have been established according to the type 
of photosynthetic pigments contained in different algal species and, to a 
lesser degree, on their external colour. Red algae, for example, contain large 
amounts of phycoerythrin and phycocyanin in addition to green chlorophylls. 
These red pigments absorb blue-green wavelengths, but reflect red. It is the 
reflected wavelengths that give algae characteristic colours. 

QUESTION 7.2 Worms are more or less elongated, relatively slender, 
soft-bodied animáis that are limbless, or nearly so. These anatomical features 
are ideally suited for living in or on soft sediments, which are saprevalent 
in the sea. Some groups (e.g. polychaetes and pogonophorans) have moved 
beyond the typical worm-like form to become more specialized species that 
are sessile tube-dwellers. 

QUESTION 7.3 There are many forms of defence against predation. Some 
examples inelude: the hard calcareous shells of clams and snails; the 
production of unpleasant compounds by sponges; the stinging nematocysts of 
cnidarians; the large spines of sea urchins; the burrowing behaviour of 
infaunal species; the cryptic, or inconspicuous, growth forms of hydroids and 

QUESTION 7.4 Using B = X x w, the biomass of clams sampled at 225 days 
is 378 m -2 x 9.910 mg, or 3746 mg m -2 . Biomass valúes calculated for all 
intervals are given below in the completed table. 


The first part of equation 5.4 considers the biomass produced and then lost 
through predation or other mortality; when this is combined with the change 
in biomass between intervals, the net production can be obtained. Thus, to 
calcúlate biomass produced, but then lost, during the interval between 50 
and 225 days, use: 

Biomass loss from the population = (X\ — X{) 

— (990 - 378) ^-364 + 9.910 ^ _ m g m ' 2 p er 175 days, 

or, divided by 175 days, biomass loss =27 mg m -2 day -1 . Combining this 
expression with the change in biomass gives the equation for net production, 

P\ = (X, - X 2 ) ( 2 V| + (B 2 - £,) 

Thus the net production in the interval between 50 and 225 days is: 

/ 5.364 + 9.910\ 

Pii5 days = (990 - 378) í--- J + (3746 - 5310) 

= 612(7.637) - 1564 = 3109.84 per 175 days, 
or P = 17.77 mg m “ 2 day -1 

Biomass losses and net production valúes for all intervals are given in the 
completed table. 

Table 7.2 Production data from a population of the clam Mactra in the North Sea. 





Mean weight 





(no. m" 2 ) 


(mg m -2 ) 

(mg m -2 day -1 ) 

(mg m 2 day -1 ) 
























12 799 




18 091 

QUESTI0N 7.5 Slow-moving or sessile invertebrates without planktonic 
larvae can be carried great distances if the eggs, young or adults are attached 
to ships or to floating objects like wood, seaweed, or bottles. Eggs of 
shallow-living species may also be transported on the feet of birds. In 
addition, the juveniles or small adults of a few snail and bivalve species can 
drift in the water column by producing long mucous threads for suspensión. 

QUESTI0N 7.6 (a) Table 4.1 lists 8 phyla with holoplanktonic members; 

Table 7.1 lists 16 phyla of marine benthos. Although the Tables ignore some 
exceptional species (e.g. there is one planktonic echinoderm and one genus 
of benthonic chaetognaths), there are many more types of benthic animáis 
than planktonic ones. 

(b) The difference can be attributed largely to the much greater physical 
variety in benthic habitats compared to the more homogeneous water column. 

QUESTION 7.7 Benthic species with lecithotrophic planktonic larvae produce 

(a) relatively few young that remain for only short times in the plankton and 

(b) therefore do not disperse far from the adult population. (c) Larval 

+ w 2 \ 



mortality is low relative to planktotrophic larvae. (d) Adult population size 
tends to be relatively constant over long intervals, (e) These factors suggest 
that the P/B ratio will be relatively low. All of the traits listed here are 
characteristic of Á^-selection. 

QUESTION 7.8 Upward growth of stromatolites takes place at about 0.5 mm 
per year. Therefore, 

1.5 m = 1500 mm 


1500 mm 
0.5 mm yr -1 

= 3000 years. 

Solving the equation gives an age of 3000 years. 


QUESTION 8.1 There is a greater water exchange bringing in more nutrients 
and plankton where the tidal range is higher. Increased nutrients lead to 
higher benthic primary production, and consequently more food for grazing 
animáis. Food also increases for benthic suspensión feeders when there are 
increased amounts of phyto- and zooplankton. Therefore the biomass per 
unit area of benthic plants and animáis should be greater in areas with a high 
tidal range. 

QUESTION 8.2 Interstitial species are very small, and body size limits the 
number of young that can be produced per individual. If fecundity is very 
low, mortality rates must also be low to ensure survival of populations. 
Direct development or brood protection ensures that the progeny remain in a 
favourable environment, and that they are not vulnerable to pelagic predators 
or filter-feeding benthos. 

QUESTION 8.3 Not necessarily. H 2 S is a by-product of sulphate reduction by 
bacteria and its occurrence, although objectionable to humans, is a natural 
process. However, H 2 S production could be indicative of pollution where 
there are large quantities of decomposing organic matter that use up oxygen. 

QUESTION 8.4 The smaller the size of the sediment particles, the greater the 
surface area per unit volume of sediment. Consequently, there is more space 
for attachment of surface-growing algae. 

QUESTION 8.5 Many freshwater and marine species are physiologically 
excluded from living in estuaries because of the salinity stress. However, for 
those that can tolérate the salinity regime, productive estuaries offer a rich 
food supply. They may also provide a more protected environment, and one 
in which there is less competition than in areas with many species. 

QUESTION 8.6 (a) The west coasts of continents are characterized by 

upwelling, and the average water temperatures are too low to permit coral 
reef growth. 

(b) These rivers discharge huge amounts of freshwater carrying very high 
loads of sediment into the sea. The resulting mixed water of reduced salinity 
and high turbidity is carried northward by prevailing currents. Both low 
salinity and high turbidity prevent or inhibit the growth of coral reefs. 


QUESTION 8.7 Yes, but these are not reef-building coráis. Ahermatypic coráis 
do not support a symbiotic relationship with zooxanthellae, and consequently 
they do not require light for nutrition and growth. 

QUESTION 8.8 Yes. The nutrient concentration in water cióse to land should 
be higher than in the middle of a tropical ocean, and consequently the barrier 
reef should have a higher productivity. 

QUESTION 8.9 The net primary productivity of mangrove swamps 
(350-500 g C m' 2 yr -1 ) is much higher than that of phytoplankton 
productivity in open tropical waters, the latter being about 75 g C m -2 yr' 1 . 

QUESTION 8.10 Large sponges are epifaunal, attaching to firm substrates, and 
all are suspensión feeders that create water currents to filter small plankton 
and bacteria from the water. The combination of hard substrates and 
abundant suspended organic partióles most frequently occurs in shallower 
areas, so it is not surprising that such sponges are most successful above 
2000 m. Sea cucumbers are either epifaunal or infaunal, depending on the 
species, but most are deposit feeders or detritivores; thus this group can 
successfully live on, or in, deep-sea soft substrates, where they obtain 
sufficient nourishment from detritus or organic material in the sediments. 

QUESTION 8.11 The biomass of both benthos and zooplankton diminishes 
rapidly with increasing depth. In both groups, biomass declines by one to 
two orders of magnitude from the surface to 1000 m, then decreases by 
another order of magnitude from 1000 m to 4000 m. 

QUESTION 8.12 Assuming that a dead diatom cell sinks at twice the rate of a 
living cell and thus at a máximum rate of about 60 m per day, it would take 
95 days to reach 5700 m depth. In actuality, sinking cells are often 
aggregated in mucoid masses, and the sinking rate would be somewhat faster 
because of the larger size of the aggregate. 

QUESTION 8.13 Except for size, Tindaria has many of the characteristics of a 
íf-selected species. It lives in a predictable environment of constant 
temperature and salinity, and it is slow-growing, reaches sexual maturity 
very late, and has a long life span. The small size of Tindaria indicates that 
it produces few young. The P/B ratio for Tindaria would be low. In 
comparison, wood-boring clams are good examples of r-selected species 
based on the characteristics described in the text. 

QUESTION 8.14 The temperature at 2500 m depth would usually be between 
2 and 4°C. 

QUESTION 8.15 Biomass is typically <20 g rrf 2 at 2500 m. The biomass of 
the Galápagos Vent community is 20-30 kg m' 2 , or 2000-3000 times 
higher. At all hydrothermal vent sites so far studied, the biomass is at least 
500-1000 times greater than in normal deep-sea communities. 

QUESTION 8.16 Both offer a great deal of cover for small animáis. Many 
animáis specifically adapted to these communities live in their shelter and 
derive a protective refuge from predators, such as birds and migrating fish. 

QUESTION 8.17 Although some individuáis do lose the competition for space 
with faster growing species, the system is usually kept in a State of flux by 
various physical and biological factors. For example, intense grazing by 
herbivores reduces algal standing stock, and predators remove animáis from 


the reef. Faster-growing organisms may face heavier predation because it is 
more efficient for animáis to consume more abundant, rather than less 
abundant food. Grazing animáis also may remove encrusting organisms, thus 
continually creating new space for settling larvae or algal spores. Success of 
larval recruitment for each species on the reef may also change over time, 
thus keeping the populations in a State of dynamic flux. Fluctuations in the 
physical environment too, such as temperature or salinity changes or 
storm-related variability, will favour certain species at one time and other 
species at other times. 

QUESTION 8.18 A sea level rise of 2 m per 100 years is equivalent to 2 cm 
per year, and at least some coráis and coral reefs are capable of such growth 
rates. Increased water depth and concomitant elevations in seawater 
temperature could affect growth and survival of some coral species. It might 
be well to keep in mind, however, that reef coráis have survived for over 
250 million years in spite of much larger sea level changes during this 
geologic period. 

QUESTION 8.19 Both communities are found in upper intertidal zones with 
fluctuating salinity, and both are dominated by salt-tolerant, erect, flowering 
plants (angiosperms) with aerial storage of biomass. The roots of mangroves 
and marshgrasses facilítate sedimentation by slowing water velocities and by 
retaining sediment, and thus they stabilize coastlines. These plants contain 
large quantities of refractory structural elements that are difficult for 
herbivores to digest, and consequently both of these communities have 
detritus-based food chains. In addition, both communities support some 
terrestrial animáis as well as marine species. 

QUESTION 8.20 Crustaceans are among the most abundant of zooplankton, 
and all of them moult their exoskeletons between each growth increment. 
Copepods, for example, have twelve distinct life stages that are separated by 
moulting, so each copepod that reaches maturity has contributed 11 moults 
to the downward flux of organic material. Overall, crustacean moults are 
extremely abundant materials in the water column. However, their nutritive 
valué for animáis is considered to be low; most are probably converted to 
bacterial biomass through the actions of chitinoclastic bacteria. 

QUESTION 8.21 Although many organisms have a limited food supply and 
live under very high pressures, there are some other properties of the deep 
sea which are advantageous to life. These inelude constant darkness, which 
helps in predator avoidance; constant temperature, which eliminates 
metabolic adjustments to change; and constant salinity, which eliminates the 
need for osmotic adjustments to varying salt concentration. 

QUESTION 8.22 Chemosynthetic bacteria appeared before photosynthetic 
algae, and the earliest marine food chains must have been based on 

QUESTION 8.23 (a) Hadal trenches, hydrothermal vents, and coid sulphide 

seeps all have high proportions of endemic species that are not found outside 
the particular community. 

(b) All of these communities are spatially isolated from similar habitats; for 
example, hydrothermal vents may be separated by hundreds of kilometres. 
Thus dispersal success may be very limited and the fauna of these 
communities tends to be isolated. As well, all of these communities have 

special environmental conditions that require a high degree of adaptation, 
and many animáis may be excluded from the sites by these constraints. 


QUESTION 9.1 Coid environments with little wave action would have 
relatively slow rates of recovery from large oil spills. This is because 
organisms in such habitats typically have slow growth rates and long life 
cycles, and because the oil will not be dispersed as quickly where there is 
little wave action and little turbulence. As well, biological degradation of oil 
proceeds more slowly at low temperatures. 

QUESTION 9.2 One obvious way is to recycle plástic products, other ways 
are to restrict dumping of trash at sea from commercial and fishing vessels 
and to prohibit the spillage of pellets from plastics factories and during 
shipment. The development and use of new types of biodegradable plástic 
should also be encouraged. 

QUESTION 9.3 The greatest impact would probably occur in summer. This is 
because the power demand (for air conditioning) is highest, while the 
temperature of ambient receiving waters is maximal. For many organisms, 
the sea temperature at that time is already near their upper thermal limit, and 
the additional heat may exceed this limit. 

QUESTION 9.4 There are only a few, one being the Mackenzie River in 
Cañada that empties into the Arctic Ocean. There are also several large 
rivers in S iberia that enter the Arctic Ocean, but at least some of these carry 
pollutants from the hinterlands. 

QUESTION 9.5 Some other anthropogenic impacts on marine organisms 
inelude changes in Coastal salinity due to dams; the impacts of offshore 
constructions such as oil rigs; the effeets of tourists on beach ecology. There 
are also beneficial changes brought about by humans including the 
construction of artificial reefs and mariculture. 

QUESTION 9.6 Probably not, but it is possible to minimize impacts by 
reducing fish harvests to some sustainable level. 

QUESTION 9.7 Sewage disposal may have beneficial effeets. In spite of the 
unwanted impacts, sewage contains nutrients that can potentially enrich 
oligotrophic waters and enhance food chains. However, such enrichment 
should not be made in confined areas such as coral atolls, where enhanced 
growth of phytoplankton, benthic algae, and seagrasses may cause coral 

QUESTION 9.8 Because some pollutants are adsorbed onto sediment partióles 
which settle on the seabed, Estuary A would have lower concentrations of 
pollutants in the water, but higher levels of pollutants in the sediments. 


Abiotic factors Nonbiological; referring to Chemical, physical and 
geological features of the environment. 

Abyssal zone The benthic zone between about 2000 m and 6000 m depth. 

Abyssopelagic zone The water column between 4000 m and 6000 m depth. 

Accessory pigments Plant pigments other than chlorophyll that capture 
photons of light used in photosynthesis; e.g. carotenes, xanthophylls, and 

Adsorption The adherence of ions or molecules in a fluid to the surfaces of 
particles suspended in the fluid. 

Advection Horizontal or vertical movement of water. 

Aerobic Living in oxygenated conditions. 

Agnatha The class of primitive fish that ineludes hagfish and lampreys. 

Ahermatypic coráis Non-reef-building coráis that lack symbiotic 

Algae A diverse group of marine plants ranging from unicellular planktonic 
species to large benthic seaweeds; all lack true roots, stems, and leaves, and 
none produce flowers or seeds. 

Amphipods Laterally compressed, planktonic or benthic crustaceans. 

Anadromous Referring to fish that breed in freshwater but spend most of 
their adult life in the sea. 

Anaerobic Living in the absence of oxygen. 

Angiosperms Flowering plants, including species of mangroves, 
marshgrasses, and seagrasses. 

Anoxic Without oxygen. 

Anthozoa A class of the Phylum Cnidaria that ineludes sea anemones and 

Anticyclonic Moving in a clockwise direction in the Northern Hemisphere 
and in an anticlockwise direction in the Southern Hemisphere. 

Aphotic zone That part of the ocean in which sunlight is absent. 

Appendicularia See Larvacea. 

Aragonite A form of calcium carbonate present in shells of pteropods and 
heteropods and in coral skeletons. 

Ascidiacea See Tünicates. 

Assimilated food That portion of ingested food that is absorbed and utilized 
by an animal, the remainder being discarded as faeces. 

Assimilation efficiency The percentage of ingested food that is assimilated 
by an animal. 

Assimilation índex A measure of primary productivity in which plant 
growth is expressed in terms of amount of carbón fixed per unit of 
chlorophyll a per unit time. 

Atoll A type of coral reef that grows around a subsided island and endoses 
a shallow lagoon. 

Attenuation A decrease in the energy of light due to absorption and 
scattering in the water column. 

Autotroph(ic) Referring to organisms that synthesize their own organic 
material from inorganic compounds; also known as primary producers. 

Auxospore A reproductive cell of diatoms that re-establishes the initial size 
of the species after a period of asexual división. 

Auxotroph(ic) Phytoplankton that require certain organic compounds, such 
as vitamins, for growth. 

Bacillariophyceae The algal class of diatoms. 

Bacterioplankton Planktonic bacteria. 

Bactivores Animáis that feed primarily on bacteria. 

Baleen whales Those species that use specialized plates of homy material 
(baleen) to filter-feed. 

Barnacle A type of benthic filter-feeding crustacean having calcareous plates 
and living permanently attached to a substrate. 

Barrier reef A type of coral reef that lies some distance offshore, with 
water between the reef and land. 

Bathyal zone The benthic zone between 200 m and about 2000 m depth. 
Bathypelagic zone The water column between 1000 m and 4000 m depth. 
Benthic Pertaining to the seafloor environment. 

Benthic boundary layer The layer of water immediately above the seafloor 
and extending upward from ten to several hundred metres above the bottom. 

Benthos Plants or animáis that inhabit the benthic environment. 

Bioaccumulation The build-up over time of substances (e.g. metáis, 
chlorinated hydrocarbons) that cannot be excreted by an organism. 

Bioerosion The breakdown of substrates, such as the calcium carbonate of 
coral reefs, by a variety of living organisms. 

Biological indicators Pelagic organisms that live within relatively narrow 
temperatura-salinity ranges and whose presence is indicative of a specific 
water mass with those environmental characteristics. 

Bioluminescence The production of light by living organisms. 

Biomagnification The increased tissue concentration of a bioaccumulated 
pollutant that occurs at successive trophic levels, resulting in top-level 
predators having the highest concentrations of substances like chlorinated 


Biomass The number of individual organisms (in some area or volume or 
región) multiplied by the average weight of the individuáis. 

Biotic factors Biological; referring to environmental influences that arise 
from the activities of living organisms, such as competing species, 
predators, etc. 

Bioturbation Disturbance of soft sediments by the movements and feeding 
activities of infauna. 

Bipolar species Those species that live in both Antarctic and Arctic waters, 
but are not present in mid-latitudes. 

Bivalves Molluscs with the shell divided into two valves; e.g. clams, 

Bloom The sudden appearance of a high concentration of phytoplankton 
resulting from increased reproduction as a response to favourable conditions. 

Blue-green algae (or bacteria) Photosynthetic organisms belonging to the 

Brachiopoda A phylum of benthic, sessile, filter-feeding animáis with a 
bi val ved calcareous shell. 

Brackish water Water of reduced salinity resulting from a mixture of 
freshwater and seawater. 

Bryozoa A group of sessile colonial animáis belonging to the Phylum 

By-catch Unwanted marine animáis that are caught incidentally by 
commercial fisheries operations. 

Calcification The process whereby calcium and carbonate ions are combined 
to form calcareous skeletal materials. 

Calcite A form of calcium carbonate present in the shells of Foraminifera 
and most benthic molluscs. 

Calorie The quantity of heat required to raise the temperature of 1 g of 
water through one Centigrade degree at 15°C. 

Carnivore An animal that feeds exclusively or primarily on other animáis. 

Catadromous Referring to fish that breed in the sea but spend most of their 
adult life in freshwater (e.g. eels). 

Catch-per-unit-effort (CPUE) The amount of ñsh caught for a given 
amount of fishing effort. 

Cetácea The order of marine mammals that ineludes whales, porpoises, and 

Chaetognaths An animal phylum of holoplanktonic, unsegmented ‘arrow 

Chemoautotrophs Bacteria that utilize the energy contained in such 
compounds as methane and hydrogen sulphide to reduce carbón dioxide and 
make organic material. 


Chemosynthesis The fixation of carbón from CO 2 into organic compounds 
by using energy derived from the oxidation of inorganic compounds such as 
ammonia, methane, and sulphur. 

Chitin A horny substance forming the hard part of crustacean exoskeletons; 
biochemically, a polymer of the carbohydrate glucosamine. 

Chitinoclastic bacteria Those that decompose chitin. 

Chlorophyll A group of green plant pigments that capture photons of light 
to be used in photosynthesis. 

Chlorophyll máximum The depth at which the concentration of chlorophyll 
is highest per unit volume of water. 

Chondrichthyes The class of fish that ineludes skates, rays, and sharks. 

Chordata A phylum of animáis that ineludes planktonic salps and 
larvaceans, benthic tunicates and vertebrates. 

Ciguatera fish poisoning (CFP) An illness common in tropical and 
subtropical countries, acquired by eating fish that have accumulated toxins 
from dinoflagellates attached to seaweeds. 

Ciliates Planktonic or benthic protozoans that have hair-like structures called 
cilia which are used for locomotion and, in some species, for feeding. 

Cladocera Planktonic crustácea with a bivalved exoskeleton. 

Cnidaria A phylum that ineludes jellyfish, sea anemones, and coráis; 
formerly Coelenterata. 

Coccolithophorids Small, flagellate, unicellular phytoplankton having 
calcareous plates (coccoliths) in their cell walls. 

Coelenterates Animáis of the Phylum Cnidaria. 

Cohort A group of organisms produced at the same time; one generation. 

Coid core ring A rotating body of water with a relatively cool temperature 
and high productivity. 

Community An ecological unit composed of the various populations of 
micro-organisms, plants, and animáis that inhabit a particular area. 

Compensation depth The depth at which the amount of carbón fixed in 
organic material by photosynthesis is equal to that which is consumed by the 
plants during respiration over a 24-hour period; also, the lower boundary of 
the euphotic zone. 

Compensation light intensity The amount of light at which photosynthetic 
production just balances respiratory losses in the plants. 

Competition The interaction among organisms that results when a necessary 
resource is in limited supply. 

Continental shelf The zone bordering a continent, extending from the line 
of permanent immersion to the depth (usually 200 m) at which there is a 
marked increase in the slope. 

Continental slope The relatively steep downward slope from the outer edge 
of the continental shelf to the fíat ocean floor. 


Convergence The situation in which different water masses come together, 
usually resulting in the sinking of surface water. 

Copepodites Life stages of copepods following the naupliar larvae; 
copepodite VI is the adult stage. 

Copepods A group of small planktonic, benthic or parasitic crustaceans; 
holoplanktonic species are usually the numerically dominant group of 
zooplankton captured by nets in most marine areas. 

Coral A benthic animal (often colonial) belonging to the Phylum Cnidaria, 
Class Anthozoa, that forms a calcareous exoskeleton. 

Cosmopolitan species Those species with a very broad geographical 
distribution; present in extensive areas of the Atlantic, Pacific, and Indian 

Countershading Colour difference in the dorsal and ventral surfaces of an 
animal; a protective mechanism against visual predators. 

Crinoids A class of echinoderms that ineludes feather stars and sea filies. 

Critical depth The depth at which the total photosynthetic production taking 
place in the water column (from the sea surface to the critical depth) is just 
balanced by the total respiratory losses of the phytoplankton within the same 
depth layer. 

Critical phase That period in the life of a fish between hatching and the 
absorption of the yolk sac. 

Crustácea A large class of primarily aquatic arthropods characterized by 
having a segmented body, paired appendages, and a chitinous exoskeleton. 

Ctenophores Gelatinous zooplankton having eight longitudinal rows of 
fused cilia fetenes’) used in swimming. 

Cyanobacteria A class of photosynthetic organisms, some of which are 
capable of nitrogen-fixation. 

Cyclonic Moving in a counterclockwise direction in the Northern 
Hemisphere and in a clockwise direction in the Southern Hemisphere. 

Cypris A larval stage of barnacles that succeeds the naupliar stages. 

Cysts Dormant stages of dinoflagellates that germinate under favourable 
conditions to produce swimming cells. 

DDE Dichloro-diphenyl-ethane, a breakdown derivative of the 
pesticide DDT. 

DDT The Chemical pesticide dichloro-diphenyl-trichloro-ethane. 

Decapods A group of large crustaceans that ineludes crabs, lobsters, shrimp, 
and prawns. 

Decomposer An organism that breaks down dead organic material to 
inorganic forms. 

Decomposition The breakdown of organic materials into inorganic elements 
by the mediation of bacteria. 


Deep scattering layer A sound-reflecting layer caused by aggregations of 

Demersal Pelagic species that live near the seafloor; see also epibenthic. 

Denitrification The formation of reduced nitrogen compounds from nitrate. 

Density In physical terms, mass per unit volume; in ecological terms, 
numbers of individuáis per unit volume or area. 

Deposit feeding Feeding on organic particles located on or in the sediments 
of the seafloor. 

Detritivores Animáis that feed primarily on detritus. 

Detritus Organic debris. 

Diapause A period of suspended development or growth, accompanied by 
greatly decreased metabolism. 

Diatomaceous ooze A sediment in which at least 30% of the particles are 
the skeletal remains of diatoms. 

Diatoms Unicellular phytoplankton with an extemal skeleton of silica. 

Diel Referring to events that occur with a 24-hour periodicity. 

Diel vertical migration The vertical migration of pelagic species that occurs 
with a 24-hour periodicity. 

Dinoflagellates Unicellular plankton having two flagella and, in some 
species, a cellulose test. 

Disphotic zone The area of low light lying between the euphotic and aphotic 

Dissolution The breakdown of calcareous skeletal material to dissolved 
calcium and carbonate ions. 

Diurnal Referring to events that occur during daytime. 

Diurnal tide A tide with one high water and one low water each tidal day. 

Divergence The horizontal flow of water away from a coast or away from a 
common centre, usually resulting in upwelling. 

Diversity See Species diversity. 

DOM Abbreviation for dissolved organic matter. 

Domoic acid A neurotoxin produced by the diatom Pseudonitzschia. 

Doubling time The time required for a population to double in size. 

Downwelling The sinking of water. 

Echinodermata A phylum of marine animáis that ineludes starfish, sea 
urchins, and sand dollars among others. 

Echinoids A class of echinoderms that ineludes sea urchins and sand dollars. 
Echiura A phylum of benthic marine worms (echiurids). 


Ecological efficiency The amount of energy extracted from a given trophic 
level divided by the energy supplied to that trophic level. 

Ecosystem An ecological unit composed of the abiotic environment, 
together with one or more communities of organisms living in a large 
geographic area. 

Eddy A circular movement of water. 

Egestion The voiding of unutiiized food as faeces. 

El Niño Episodio climatic changes that inelude warming of the equatorial 
Pacific Ocean, and suppression of upwelling into the euphotic zone off the 
coast of Perú by intrusions of this warm, nutrient-poor, surface water. 

Elasmobranchs Fish with cartilaginous skeletons; sharks, skates, and rays. 

Endemic Organisms restricted to specific habitats. 

Enteropneusts Benthic marine worms belonging to the Phylum 

Entrainment Mixing of salí water into fresh water, as in an estuary. 

Environment A collective term for the conditions in which an organism 
lives, including abiotic features (e.g. light, temperature) and biotic features 
(e.g. predators, competitors). 

Epibenthic Referring to pelagic species that live in association with the 

Epifauna Animáis that live on, or attach to, a substrate surface. 

Epipelagic zone The upper región of the sea from the surface to about 
200-300 m depth. 

Epiphytes Plants that grow on the surfaces of other plants. 

Epipsammic algae Those species that live on sand grains. 

Epontic algae Algae that grow within sea ice. 

Equilibrium species Those species that are usually of relatively large size, 
have slow growth rates and long lifie spans, produce few young, and have 
relatively constant population sizes that are at or near the carrying capacity 
of the environment in which they live; AT-selected species. 

Estuary A semi-isolated Coastal area that is diluted by freshwater discharge. 

Euphausiids Shrimplike, holoplanktonic crustaceans; ‘krilF. 

Euphotic zone The surface waters of the oceans that receive sufficient light 
to support photosynthesis. 

Eurybathic Able to tolérate a wide range in depth (pressure). 

Euryhaline Able to tolérate a wide range in salinity. 

Eurythermic Able to tolérate a wide temperature range. 

Eutrophic Referrihg to areas that contain high nutrient concentrations and 
support high biological productivity. 


Excretion The elimination of wastes produced from metabolic processes, 
usually in the form of urea or ammonia. 

Extinction coefficient The ratio between the intensity of light at a given 
depth and the intensity at the sea surface. 

Exudation The release of dissolved metabolites by phytoplankton. 

/-ratio The ratio of new production to total (new + regenerated) production. 
Fecundity The rate of production of eggs or young. 

Femtoplankton Planktonic organisms (viruses) of 0.02-0.2 ¡um. 

Filter feeding See Suspensión feeding. 

Food chain A linear sequence of organisms in which each is food for the 
next member in the sequence. 

Food web A schematic depiction of the feeding interactions in a community. 

Foraminifera Planktonic or benthic protozoans with a calcareous 
exoskeleton and pseudopodia. 

Foraminiferan ooze A sediment containing 30% or more foraminiferan 

Fringing reef A type of coral reef attached directly to a land mass and not 
separated from it by a lagoon. 

Frustule The extemal skeleton of a diatom. 

Gelatinous zooplankton Fragüe planktonic animáis without rigid 
exoskeletons and with high water concentrations in their gelatinous tissues; 
e.g., ‘jelly’fish, siphonophores, ctenophores, salps. 

Generation time The number of generations produced per unit time. 

Gramme carbón (g C) An expression of biomass in terms of the weight of 
carbón in a sample. 

Grazing The consumption of plants by herbivores. 

Gross photosynthesis The total amount of photosynthetic production before 
subtracting losses due to respiration. 

Gross primary production The total quantity of organic tissue (or of 
carbón) fixed by photosynthesis. 

Growth effieiency The amount of growth attained per unit of ingested (gross 
growth effieiency) or assimilated (net growth effieiency) food. 

Gymnosomes Holoplanktonic, shell-less, carnivorous snails. 

Gyre A circular motion of water, larger than an eddy. 

Habitat The place inhabited by a plant or animal species. 

Hada! zone The benthic zone from 6000 m to the deepest areas of the ocean. 


Hadalpelagic The pelagic zone from 6000 m to the deepest areas of the 

Half-life The length of time required for the radioactivity of a substance to 
decline by one-half. 

Halocline The zone showing the greatest change in salinity with depth. 

Herbivore An animal that feeds exclusively or primarily on plants. 

Hermaphrodite An animal that produces both male and female gametes. 

Hermatypic coral A reef-building coral that contains zooxanthellae in its 

Heteropods Holoplanktonic snails characterized by having a single 
swimming fin. 

Heterotroph(ic) Referring to organisms that require organic materials 
for food. 

High water The máximum height reached by a rising tide. 

HNLC areas Ocean regions with high nitrate concentration but low 
chlorophyll concentration. 

Holoplankton Planktonic organisms that spend their entire lives in the water 
column; permanent residents of the plankton community. 

Holothuroidea A class of echinoderms that ineludes sea cucumbers. 

Homoiothermic Warm-blooded; able to regúlate intemal body temperature. 

Hydroids Benthic colonial cnidarians, some of which produce 
free-swimming medusae. 

Hydrostatic pressure The pressure exerted at a given depth by the weight 
of the overlying column of water. 

Hypersaline Referring to water with a salinity of 40 or more. 

Ichthyoplankton Planktonic fish eggs and fish larvae. 

Infauna Animáis that live within the sediments of the seafloor. 

Infrared Invisible wavelengths of light, longer than about 780 nm, which 
are responsible for heating the ocean. 

Ingestión The act of swallowing food. 

Interstitial fauna Animáis that live in the spaces between adjacent partióles 
in a soft-bottom substrate. 

Intertidal zone The zone between high and low tide marks that is 
periodically exposed to air. 

Isopods An order of crustaceans generally having a flattened body and with 
both benthic and planktonic species. 

Isothermal Of equal temperature. 


Jellyfish See Medusae. 

tf-selection A life history pattern in which the species survive by being 
well-adapted and efficient, rather than by producing large numbers of young; 

see Equilibrium species. 

Kelp A group of very large brown algae that grow subtidally in mid- and 
high latitudes. 

Keystone species A species that maintains community structure through its 
feeding activities, and without which large changes would occur in the 

Krill Euphausiids; often specifically Euphausia superba of the Antarctic 
Ocean. The word comes from an oíd Norwegian term kril , once applied to 
such diverse animáis as vermin and larval fish. 

Lagoon A shallow body of water encircled by an atoll, or lying between a 
land mass and a barrier reef. 

Larvacea Zooplankton of the Phylum Urochordata that build houses of 
mucus and filter-feed on nanoplankton. 

Lecithotrophic larvae Meroplanktonic larvae that do not feed on 
planktonic food. 

Littoral zone The intertidal zone. 

Low water The minimum height reached by a falling tide. 

Macrobenthos See Macrofauna. 

Macrofauna Benthic animáis larger than 1.0 mm. 

Macrophytes Large, visible plants (e.g. mangroves, rock algae, seagrasses). 
Macroplankton Zooplankton of between 2 cm and 20 cm in size. 

Mangal Mangrove swamp. 

Mangroves Referring to a variety of salt-tolerant trees and shrubs that 
dominate many intertidal regions in tropical and subtropical latitudes. 

Maricuiture The artificial cultivation of marine species. 

Marine snow Detrital aggregates >0.5 mm consisting of faecal pellets, 
empty larvacean houses, pteropod feeding webs, and other materials derived 
from living organisms, as well as associated bacteria. 

Medusae Bell-shaped zooplankton of the Phylum Cnidaria; ‘jellyfish’. 

Megalopa A larval stage of crabs that follows the zoea stages. 

Megaplankton Zooplankton of between 20 cm and 200 cm in size. 

Meiobenthos See Meiofauna. 

Meiofauna Animáis between 0.1 mm and 1.0 mm in size that live in 
sediments; interstitial fauna. 


Meroplankton Plankton that spend only part of their life cycle in the water 
column, usually the eggs and larvae of benthic or nektonic adults. 

Mesopelagic zone The water column from the bottom of the epipelagic zone 
(200-300 m) to about 1000 m depth. 

Mesoplankton Plankton of between 0.2 mm and 20 mm in size. 

Mesotrophic Referring to a región with modérate concentrations of nutrients 
and modérate biological productivity. 

Microbenthos See Microfauna. 

Microbial loop Referring to the regeneration of nutrients, and their return to 
the food chain, that is mediated by bacteria and protozoans. 

Microfauna Benthic animáis smaller than 0.1 mm; mostly protozoans. 

Microphytes Microscopio benthic plants. 

Microplankton Plankton of between 20 pm and 200 pin in size. 

Mineralization The breakdown of organic compounds into inorganic 

Mixed layer A layer of water that is mixed by wind and is therefore 

Mixotrophy Employing more than one type of feeding strategy in order to 
exploit different food resources. 

Mollusca A phylum of animáis that ineludes snails, clams, and squid 
among others. 

Mucus A sticky exúdate composed mainly of proteins and polysaccharides. 
Mysids An order of shrimplike crustaceans, mostly epibenthic. 

Mysticetes Baleen whales. 

Nanoplankton Plankton in the size range of 2-20 pm, including some 
phytoplankton and some protozoans. 

Nauplius A free-swimming larval stage of crustaceans. 

Neap tides Tides occurring near the first and last quarters of the Moon, 
when the tidal range is least. 

Nekton Pelagic animáis capable of swimming against a current; adult squid, 
fish, and marine mammals. 

Nematocysts Stinging cells on the tentacles of cnidarians. 

Nemertea A phylum of unsegmented marine worms, all with a proboscis. 

Neritic Referring to inshore waters shallower than 200 m in depth that 
overlie continental shelves. 

Net photosynthesis The amount of photosynthetic production in excess of 
respiration losses. 


Net primary production That portion of the gross primary production 
which is incorporated within the body of the primary producer and thus 
appears as growth. 

Neuston Organisms that inhabit the uppermost few millimetres of the 
surface water. 

New nitrogen Nitrogen that enters the euphotic zone from outside regions, 
especially nitrate entering in upwelled water. 

New production Photosynthetic production based on new nitrogen. 

Nitrification The oxidation of ammonia to nitrite and nitrate. 

Nitrogen fixation The conversión of dissolved nitrogen gas to organic 
nitrogen compounds, usually mediated by cyanobacteria. 

Nutricline The depth zone where nutrient concentrations increase rapidly 
with depth. 

Nutrient Any of a number of inorganic or organic compounds or ions used 
primarily in the nutrition of primary producers; e.g., nitrogen and 
phosphorus compounds. 

Oceanic Referring to offshore waters in areas deeper than 200 m. 

Odonticetes Predatory toothed whales, dolphins and porpoises. 

Oligohaline Referring to water of low salinity, usually less than about 5. 

Oligotrophic Referring to a región with low nutrients and low biological 

Omnivore An animal that eats both plant and animal foods. 

Ophiuroids A class of echinoderms that ineludes brittle stars. 

Opportunistic species Those species that are usually of relatively small 
size, have rapid growth rates and short life spans, produce many young, and 
have variable population sizes that are below the carrying capacity of the 
environment in which they live; r-selected species. 

Osmoregulation Referring to physiological mechanisms that maintain the 
internal salt and fluid balance of an organism within an acceptable range. 

Osmosis The movement of water through a semipermeable membrane 
separating two Solutions of differing solute concentrations, making the 
concentration of water equal on both sides of the membrane. 

Osteichthyes The class of fish that ineludes all those species with bony 
skeletons; the teleosts. 

Ostracods A class of Crustácea characterized by having a bivalved 

Overfishing When the quantity of fish harvested exceeds the amount that 
can be resupplied by growth and reproduction. 

Oxygen mínimum layer A zone of low oxygen concentration, usually 
between 400 m and 800 m depth. 


PAR Abbreviation for photosynthetieally active radiation; wavelengths of 
between about 400 nm and 700 nm that are used by plants in photosynthesis. 

Paralytic shellfish poisoning (PSP) A sometimes fatal paralysis resulting 
from the ingestión of shellfish containing saxitoxin, a neurotoxin produced 
by certain dinoflagellates and acquired by the shellfish feeding on these 
toxic algae. 

Parthenogenesis Reproduction without fertilization, resulting in cloned 

Patchiness A spatial pattern in which individuáis are not distributed either 
uniformly or randomly, but are clustered in ‘patches’ of variable size. 

P/B ratio The ratio of annual production to average annual biomass for a 
particular species; high ratios are indicative of highly productive short-lived 
organisms (e.g. phytoplankton), and low ratios indicate large, slow-growing 
organisms (e.g. fish). 

Pelagic Referring to the ocean water column and the organisms living 

Photocyte A cell in which bioluminescent light is produced. 

Photoinhibition The inhibition of photosynthesis by high light intensities. 

Photophore A complex organ in which bioluminescent light is produced. 

Photosynthesis The process whereby plants utilize carbón dioxide, water, 
and solar energy to manufacture energy-rich organic compounds. 

Photosynthetic quotient The amount (in moles) of O 2 produced by 
photosynthesis divided by the amount (in moles) of CO 2 taken up in the 

Phytodetritus Nonliving particulate matter derived from phytoplankton or 
benthic plants. 

Phytoplankton Microscopic planktonic plants; e.g. diatoms, dinoflagellates. 

Picoplankton Plankton measuring 0.2-2.0 pm in size, mostly bacteria. 

Pinnipedia The order of marine mammals that ineludes the seáis, sea lions, 
and walruses, all having four swimming flippers. 

Plankton Plants or animáis that live in the water column and are incapable 
of swimming against a current. 

Planktotrophic larvae Meroplanktonic larvae that depend on feeding on 
planktonic food (e.g. bacteria, phytoplankton) for their growth. 

Pleuston Organisms that float passively at the sea surface and whose bodies 
project partly into the air. 

POC Abbreviation for particulate organic carbón. 

Pogonophora A phylum of benthic marine worms, all of which lack a 
mouth and gut. 

Poikilothermic Cold-blooded; unable to regúlate body temperature. 


Polar The Arctic or Antarctic regions. 

Polychaetes Marine segmented worms belonging to the Phylum Annelida; 
some are planktonic, but most are benthic. 

Polyp An individual organism of a colonial cnidarian such as a hydroid or 
coral colony. 

Population All the individuáis of one species that inhabit the same 
geographic area. 

Population density The number of individuáis per unit area (or per unit 
volume of water) in a population. 

Predation The act of an animal feeding upon another animal. 

Primary consumers Herbivorous animáis. 

Primary producers Plants. 

Primary production The amount of organic material synthesized from 
inorganic substances per unit volume of water or unit area. 

Prochlorophytes Photosynthetic picoplankton lacking a nucleus and closely 
related to Cyanobacteria. 

Production That part of assimilated energy which is retained and 
incorporated in the biomass of the organism. 

Productivity The rate at which a given quantity of organic material is 
produced by organisms. 

Protists A collective term for unicellular organisms that have cells with a 
true nucleus, including diatoms, dinoflagellates and protozoans. 

Protozoa Unicellular animáis. 

Pteropod A holoplanktonic snail having two swimming wings. 

Pteropod ooze Seafloor sediments composed of more than 30% CaCC >3 
from pteropod or other pelagic mollusc shells. 

Pycnocline The water layer in which density changes most rapidly with 

r-selection A life history pattern in which the species survive by producing 
very large numbers of offspring, and opportunistically move into new 
suitable habitats whenever they become available; see Opportunistic 

Radiolaria Planktonic protozoans with a siliceous skeleton and pseudopodia. 

Radiolarian ooze A sediment formed from the remains of radiolarian 

Recruitment The addition of new (juvenile) individuáis to a population. 

Red tide A red coloration of seawater caused by high concentrations of 
certain species of micro-organisms, usually dinoflagellates, some of which 
release toxins. 


Refractory materials Those that are resistant to decomposition. 

Regenerated production Photosynthetic production based on nitrogen that 
is recycled within the euphotic zone. 

Regime shift A long-term change in marine ecosystems and/or in biological 
production resulting from a change in the physical environment. 

Respiration A metabolic process carried out by all organisms in which 
organic substances are broken down to yield energy; it is the opposite 
reaction to photosynthesis, and results in a release of carbón dioxide. 

Resting spore A spore formed by diatoms or dinoflagellates which remains 
dormant for some period of time before reforming an active planktonic cell. 

Salinity The total amount of dissolved material (salts) in seawater measured 
in g kg~* of seawater (formerly denoted as parts per thousand, ppt, or 
but it is a dimensionless number that is now reported without units). 

Salps Barrel-shaped gelatinous zooplankton of the Phylum Urochordata. 

Saltmarsh An intertidal community dominated by emergent vegetation 
rooted in soils. 

Saxitoxin A collective term for various neurotoxins produced by certain 
species of dinoflagellates. 

Seagrass A collective ñame for certain marine flowering plants that grow in 
intertidal soft-substrate communities. 

Seamount A submerged, isolated, mountain that rises from the seafloor. 
Secondary consumers Carnivorous animáis. 

Secondary production The amount of organic material produced by animáis 
from ingested food. 

Self-shading The reduction in light caused by increasing numbers of 

Semidiurnal tide A tide with two high waters and two low waters each 
tidal day. 

Sessile Referring to animáis that are permanently attached to a substrate. 
Siliceous Containing silica. 

Silicoflagellates Small, flagellate, unicellular phytoplankton with a siliceous 

Siphonophores Pelagic colonial cnidarians. 

Sipuncula A phylum of unsegmented marine worms, mostly benthic. 

Sirenia The order of herbivorous marine mammals that ineludes manatees 
and dugongs. 

Species A distinctive group of interbreeding individuáis. 

Species diversity The number of species in a particular area; or a measure 
derived from combining the number of species with their relative abundance 
in an area. 


Species succession Successive changes in the relative abundance of species 
in a community which result from environmental change. 

Spreading centre A región along which new seafloor is being produced. 

Spring tides Tides occurring near the times of the new and full moon, when 
the tidal range is greatest. 

Standing crop See Standing stock. 

Standing stock The biomass of organisms present per unit volume or per 
unit area at a given time. 

Stenobathic Referring to organisms that can tolérate only a narrow depth 
(pressure) range. 

Stenohaline Referring to organisms that can tolérate only a narrow salinity 

Stenothermic Referring to organisms that can tolérate only a narrow 
temperature range. 

Stock/recruitment theory Fisheries management theories based on the 
relation between the numbers of adult fish and the predicted numbers of 
juvenile fish that enter the adult population. 

Sublittoral zone The benthic zone extending from the low tide mark to the 
outer edge of the continental shelf. 

Substrate A solid surface on which an organism Uves or to which it is 
attached (also called substratum); or, a Chemical that forms the basis of a 
biochemical reaction or acts as a nutrient for microorganisms. 

Subtidal zone See Sublittoral zone. 

Supralittoral zone The narrow benthic zone just above high water mark, 
immersed only during storms. 

Supratidal zone See Supralittoral zone. 

Suspensión feeding Obtaining food by filtering particles out of the 
surrounding water. 

Symbiosis A cióse physiological association between two species, often for 
mutual benefit. 

Tanaids Small benthic marine crustaceans. 

Teleosts Fish with bony skeletons. 

Thecosomes Holoplanktonic snails having paired swimming wings and 
usually a calcareous shell. 

Thermocline The water layer in which temperature changes most rapidly 
with increasing depth. 

Tidal range The difference in height between consecutive high and low 

Tide The periodic rise and fall of the sea surface due to gravitational 
attractions of the Sun and Moon acting on the rotating Earth. 

Tintinnids Planktonic, ciliate protozoans having a proteinaceous outer shell. 


Top-level predators Animáis that have no natural predators, other than 

Transfer efficiency The annual production in one trophic level divided by 
the annual production in the preceding (lower) trophic level; a measure of 
the efficiency with which energy is transferred between trophic levels. 

Trench A narrow, relatively steep-sided depression in the seafloor that lies 
below 6000 m in depth. 

Trochophore A free-swimming larval stage of polychaete worms and some 
molluscs, characterized by having bands of cilia around the body. 

Trophic level The nutritional position occupied by an organism in a food 
chain or food web; e.g. primary producers (plants); primary consumers 
(herbivores); secondary consumers (carnivores), etc. 

Tunicates Sessile benthic animáis belonging to the Phylum Chordata. 

Turbidity Reduced visibility in water due to the presence of suspended 

Turbulence Physical mixing of water. 

Ultraviolet Invisible radiation with wavelengths of less than 380 nm. 
Upwelling A rising of nutrient-rich water toward the sea surface. 

Veliger A free-swimming larval stage of molluscs. 

Vestimentifera A group of benthic marine worms related to the Pogonophora 
and characteristically found at hydrothermal venís and coid seeps. 

Visible spectrum Visible radiation with wavelengths of approximately 
400-700 nm. 

Warm core ring A rotating body of water with relatively warm temperature 
and low productivity. 

Water mass A large volume of seawater having a common origin and a 
distinctive combination of temperature, salinity, and density characteristics. 

Xenophyophoria Large, unicellular, benthic protists. 

Zoea A planktonic larval stage of crabs with characteristic spines on the 

Zonation Parallel bands of distinctive plant and animal associations that 
develop in intertidal regions. 

Zoobenthos Benthic animáis. 

Zooflagellates Colourless, heterotrophic, flagellated protists. 

Zooplankton Planktonic animáis. 

Zooxanthellae Photosynthetic micro-organisms, usually dinoflagellates, that 
live symbiotically in the tissues of organisms such as coráis and molluscs. 


We wish to gratefully acknowledge the assistance of The Open University 
oceanography course team in preparing this volume: Angela Colling, John 
Phillips, Dave Park, Dave Rothery, John Wright. They generously passed on 
their experience in writing previous volumes in this series, and their advice 
and critiques of our early drafts were invaluable. 

Colour figures were generously provided by Dr. F. J. R. Taylor, University of 
British Columbia (Colour Plates 1-3, 5, 6, 9-11, 38); Suisan Aviation Co., 
Tokyo t Píate 4); NSF/NASA {Plates 7, 8 ); Dr. L. P. Madin, Woods Hole 
Oceanographic Institution {Plates 12, 14, 15, 25-27)', Dr. G. R. Harbison, 
Woods Hole Oceanographic Institution {Plates 13, 22); R. W. Gilmer 
{Plates 16, 17, 20, 21, 28); Dr. M. Omori, Tokyo University of Fisheries 
{Píate 23); Dr. A. Alldredge, University of California, Santa Barbara 
{Píate 24 taken by J. M. King); Dr. T. Carefoot, University of British 
Columbia {Plates 31, 36, 37); Department of Energy, Mines and Resources, 
Cañada {Píate 33); Dr. J. B. Lewis, McGill University {Plates 34, 35); 

Dr. R. R. Hessler, Scripps Institution of Oceanography {Piafes 39, 40); and 
P. Lasserre, Station Biologique de Roscoff {Píate 41 ). 

In-text photographs were kindly provided by Dr. F. J. R. Taylor, University 
of British Columbia {Figure 3.3d); R. Gilmer {Figure 4.3); Dr. O. Roger 
Anderson, Lamont-Doherty Geological Observatory of Columbia University 
{Figures 4.5, 4.6); R. Brown, Department of Fisheries and Oceans, Cañada 
{Figure 4.17); Fisheries and Oceans (Cañada) {Figure 6.17); C. E. Mills, 
Friday Harbor Laboratories {Figure 9.3); B. E. Brown, University of 
Newcastle upon Tyne {Figure 9.4 ). 

We are extremely grateful to Mrs. Barbara Rokeby who patiently produced 
the many drafts of figures and who contributed several original drawings. 
The following fine figures were reprinted or modified and redrawn from 
previously published material, and grateful acknowledgement is made to the 
following sources: 

Figure 1.2 J. McN. Sieburth et al. (1978) in Limnology and Oceanography, 
23, American Society of Limnology and Oceanography; Figures 1.5, 1.6 

C. W. Thomson and J. Murray (1885) Report on the Scientific Results of the 
Voyage ofHMS Challenger during the years 1873-76, Narrative , Vol. I, First 
Part; Figure 1.7 E. Haeckel (1887) Report on the Scientific Results of the 
Voyage ofHMS Challenger during the years 1873-76, Zoology, Vol. XVIII; 
Figure 2.5 G. L. Clarke and E. J. Dentón (1962) in The Sea, Ideas and 
Observations on Progress in the Study of the Seas, Interscience; Figure 2.6, 
The Open University (1989) Ocean Circulation, Pergamon; Figures 2.7, 2.9, 
2.11, 2.14, 2.16, 2.18, 6.6 The Open University (1989) Seawater: Its 
Composition, Properties and Behaviour, Pergamon; Figure 2.10 R. V. Tait 
(1968) Elements of Marine Ecology , Butterworths Scientific Ltd; Figure 2.19 
A. N. Strahler (1963) Earth Sciences, Harper & Row Pubs; Figures 2.13, 
2.15 W. J. Emery & J. Meinke (1986) Oceanographica Acta, 9, Gauthier 
Villars; Figure 2.12 H. V. Sverdrup et al. (1942) The Oceans, Prentice Hall 
Inc.; Figures 3.1a-d,i E. E. Cupp (1943) Marine Plankton Diatoms of the 
West Coast ofNorth America, University of California; Figures 3.1 e-h 

D. L. Smith (1977) A Guide to Marine Coastal Plankton and Marine 


Invertebrate Larvae , Kendall/Hunt; Figure 3.3 M. V. Lebour (1925) The 
Dinoflagellates of Northern Seas, Marine Biological Association of the U.K.; 
Figures 3.5, 3.6, 3.18, 5.9 T. R. Parsons el al. (1984) Biological 
Oceanographic Processes , Pergamon; Figure 3.8 U. Sommer (1989) in 
Plankton Ecology , Springer-Verlag; Figure 3.9 P. Tett; Figure 3.10 
T. R. Parsons (1979) in South African Journal of Science, 75, South African 
Research Council; Figures 3.13, 3.14 R. D. Pingree (1978) in Spatial 
Patterns in Plankton Communities, Plenum; Figures 3.16, 7.1, 7.2, 8.13 
(1984) Oceanography, Biological Environments, The Open University; 

Figure 3.17 A. K. Heinrich (1962) in Journal Conseil International pour 
VExploration de la Mer, 27, Conseil International pour FExploration de la 
Mer; Figure 4.7 E. N. Kozloff (1987) Marine Invertebrates of the Pacific 
Northwest , University of Washington; Figure 4.8 Zhuang Shi-de and Chen 
Xiaolin (1978) in Marine Science and Technology (in Chinese), 9, State 
Oceanographic Administration of the People’s Republic of China; 

Figure 4.10 J. Fraser (1962) Nature Adrift, Foulis; Figure 4.15 M. Omori 
(1974) in Advances in Marine Biology, 12, Academic Press; Figure 4.16 
E. Brinton (1967) in Limnology and Oceanography , 12, American Society of 
Limnology and Oceanography; Figure 4.18 J. Fulton (1973) in Journal of 
the Fisheries Research Board of Cañada , 30, Fisheries Research Board of 
Cañada; Figure 4.19 R. Williams (1985) in Marine Biology, 86, Springer 
International; Figure 4.20 D. L. Mackas et al (1985) in Bulletin of Marine 
Science, 37, University of Miami; Figure 4.21 P. H. Wiebe (1970) in 
Limnology and Oceanography, 15, American Society of Limnology and 
Oceanography; Figure 4.22 S. Nishizawa (1979) in Scientific Report to the 
Japanese Ministry of Education, No. 236017; Figure 4.23 Sir Alister Hardy 
Foundation for Ocean Science, Annual Report 1991; Figure 5.2 
R. W. Sheldon et al. (1972) in Limnology and Oceanography, 17, American 
Society of Limnology and Oceanography; Figure 5.4 A. Clarke (1988) in 
Comparative Biochemistry and Physiology, 90B, Pergamon; Figure 5.6 
J. H. Steele (1974) The Structure of Marine Ecosystems, Harvard University; 
Figure 5.8 B. C. Cho and F. Azam (1990) in Marine Ecology Progress 
Series, 63, Inter-Research; Figure 5.14 W. H. Thomas and D. L. R. Seibert 
(1977) in Bulletin of Marine Science, 27, University of Miami; Figure 5.17 
T. R. Parsons and T. A. Kessler (1986) in The Role of Freshwater Outflow in 
Coastal Marine Ecosystems , Springer-Verlag; Figure 5.18 Yu. I. Sorokin 
(1969) in Primary Productivity in Aquatic Environments, University of 
California; Figure 5.19 T. R. Parsons and P. J. Harrison (1983) in 
Encyclopedia of Plant Physiology, 12D, Springer-Verlag; Figure 5.21 
R. L. Iverson (1990) in Limnology and Oceanography, 35, American Society 
of Limnology and Oceanography; Figure 6.1 R. Payne; Figure 6.2 
N. P. Ashmole (1971) in Avian Biology, I, Academic Press; Figures 6.7a, 8.5 
Friedrich (1969) Marine Biology, Sidgwick & Jackson; Figure 6.7b, 6.8b 
N. B. Marshall (1954) Aspects of Deep Sea Biology, Hutchinson; Figure 6.8a 
C. P. Idyll (1964) Abyss, Thomas Crowell; Figure 6.9 F. S. Russell et al. 
(1971) in Nature, Macmillan; Figure 6.14 R. S. K. Barnes and R. N. Hughes 
(1988) An Introduction to Marine Ecology, Blackwell; Figure 6.15 
Department of Fisheries and Oceans, Cañada; Figure 7.3 G. Thorson (1971) 
Life in the Sea, Weidenfeld & Nicolson, by permission of the Estate of 
Gunnar Thorson; Figure 7.4 T. Fenchel (1969) in Ophelia, 6, Marine 
Biological Laboratory, Helsingoer, Denmark; Figure 8.1 J. Connell (1961) 
in Ecology, 42, Ecological Society of America; Figure 8.2 R. Paine (1966) 
in American Naturalist, 100, American Society of Naturalists; Figure 8.7 
A. Remane (1934) in Zoologischer Anzeiger, Suppl. 7,; Figure 8.9 


D. E. Ingmanson and W. J. Wallace (1973) Oceanology: An 1ntroduction , 
Wadsworth; Figure 8.15 J. W. Nybakken (1988) Marine Biology , An 
Ecological Approach, Harper & Row; Figure 8.16 R. D. Turner and 
R. A. Lutz (1984) in Oceanus, 27, Woods Hole Oceanographic Institution. 

Data in Table 5.2 were taken from Laws (1985) in American Scientist , 73, 
Sigma Xi, The Scientific Research Society; data on seabird numbers in 
Figure 6.10 were kindly provided by F. Chavez, Monterey Bay Aquarium 
Research Institute; data in Table 7.2 were taken from Birkett (1959) in 
Conseil L. International Exploration de la Mer (Unpublished Report C. M., 
No. 42); Table 8.1 is adapted from R. S. K. Barnes and R. N. Flughes 
(1988) An 1ntroduction to Marine Ecology, Blackwell. 


Note: page numbers in italics refer to illustrations; in bold to tables 

abalone 172 

above-tide forest (of mangroves) 224 
abyssal storms 226-227, 238 
abyssal zone 3, 177, 226-238 
abyssopelagic species 93 
abyssopelagic zone 3, 93 
Acanthaster 262 

accessory pigments see photosynthetic 

active transpon 28 
advection 32 
aerobic processes 137 
Agassiz, A. 10, 11, 12 
Agassiz, L. 11 

of Earth 1 

of oceans 1, 102-103, 216 
Aglantha 77 , 102 
Agnatha 157 
Alaria 179 
Alaska 156, 204 
Alaskan Gyre 61 
Alben I, Prince 11 
Aleudan Trench 228, 229-230 
Alexandrium 44 

algae, benthic 39, 134, 178-180, 

200-201, 206, 216, 218,222, 260 
see also coralline algae; kelp; 
epiphytic 178, 211 
epipsammic 178-179, 211 
epontic (ice) 117 
macrophytic 178, 235 
microphydc 179 
zonation of 179-180, 200 
algae, planktonic see phytoplankton 
Amchitka Island 203 
America (North) 44, 155, 162, 164, 172, 
174, 200, 202 

America (South, Central) 44, 155, 202 
amino acids 47, 55, 137 
ammonia 48, 55, 57, 127, 137-140 
Amoco Cádiz 252 

amphipods 77 , 83, 84, 183 , 190, 206, 
211, 228, 241 
anadromous fish 162, 250 

bacteria 136-138, 206, 211, 255, 258 
conditions/environments 44, 92, 
136-138, 206, 211, 224, 225, 
242, 255, 258 

anchovies 89, 91, 158, 163, 165-166, 

angiosperms 178 
angler-fish 160, 161-162, 235 
Anguilla 162 

Annelida/annelids 77 , 81, 183 , 186 
anoxic see anaerobio 
Antarctic Bottom Water 31, 33 
Antarctic Convergence 107 
Antarctic Divergence 62, 100 
Antarctic Intermediate Water 31, 33 
Antarctic Ocean 23, 24, 55, 84, 100, 

101, 103, 152, 154, 249 
fisheries/whaling 118, 119 , 147, 

food web 117-118, 147 
Antarctic Polar Front 63 
Antárctica 22, 24, 31, 62, 63, 100, 135 
Anthozoa 183, 185, 214 
anticyclonic gyres see gyres 
aphotic zone, definition 20, 21 
Aplacophora 183, 187, 229 
Appendicularia 77, 85 see also Larvacea 
aquaculture see mariculture 
aragonite 142 
Architeuthis 148 

Arctic Ocean 23, 68, 69, 84, 96, 101, 
103, 117, 152, 155 
Argo Merchant 252 
Aristotle 7 
Arthropoda 77, 183 
Ascidiacea/ascidians 183, 189 see also 

assimilation 127, 128, 135 
efficiency 128 

assimilation index 49, 51 , 53 
Asterias 202, 258 
Asterionella 40 
Asteroidea 183 , 187 
Atlanta 77 

Atlantic Ocean 30, 32, 34, 45, 62, 63, 

67, 68, 69, 84, 98,101-103, 108, 
148, 149, 153, 155, 156, 163, 216, 
223, 228, 230, 234, 237 
atolls 216, 219, 220 
Aulacantha 11 
Aureococcus 39 

Australia 44, 62, 63, 149, 179, 213, 258 
autotrophic production 42, 47, 74 
auxospores 41 

auxotrophic production 48, 55 

Avicennia 223 
azoic hypothesis 7 

Bacillariophyceae 39, 40 
bacteria 4, 46, 90, 112, 121-123, 
136-138, 210, 211, 224, 235, 
236-237, 239, 241, 242, 255 
anaerobio 136-138, 206, 211, 255, 

symbiotic 159, 186, 240, 241 
Baffin Bay 7 
Bahamas 179 
Baird, S. 11 
Balanus 199-200 
baleen 150 

baleen whales 150-151 

ballast water transpon 102, 257-258 

Balde Sea 27, 197 

bamacles 190, 197, 198, 199-200, 224, 

barracuda 158 

barrier reefs 219, 220 see also Great 
Banier Reef 
Bathochordaeus 92 
bathyal zone 3, 177, 226 
Bathybius haeckelii 10 
Bathymodiolus 241 
bathypelagic species 93, 101 
bathypelagic zone 3, 93 
Bay of Fundy 197 
Beagle 11, 219 
Beggiatoa 239 
Benguela Current 62, 68 
benthic boundary layer 230 
benthic environments 2-3, 177-178, 

benthic production see primary 

production; secondary production 
benthic storms see abyssal storms 
benthic-pelagic coupling 196 
benthos 178-190 see also algae, benthic; 

biomass 191-192, 208, 232-233, 240 
community structure, Controls of 
192-193, 201-202, 204-205, 


definition 3 

food supply for 69, 196, 233-236 
production see primary production; 

secondary production 
types of 178-190 


Bering Sea 106, 153 
Bering Strait 103 
Bering, V. 153 
Beroe 77 , 80 

bicarbonate ions 47, 141-142 
bioerosion 221, 262 
biogeographic zones 22-23, 100-103 
biological indicators 101, 165 
bioluminescence 93, 159, 161 

and size 113 
definition 48 
bioturbation 192 
bipinnaria larva 87 
bipolar distribution 101 
birds see seabirds 

Bivalvia/bivalves 183 , 187, 229, 256 see 
also specific types 
Black Sea 102, 136, 250, 257 
bleaching, of coral 217, 263 
blooms of phytoplankton 43-44, 45, 66, 
209 see also phytoplankton 
blue-green algae see Cyanobacteria 
blue-green bacteria see Cyanobacteria 
Bodo 77 
Bolinopsis 80 

bottom trawling 247 , 248, 250 

Brachiopoda/brachiopods 183 , 188-189 

brackish water 27 see also estuaries 

Brandt, von T. 12 

Brazil Current 34 

British Columbia 130, 204 

brittle stars 187, 188 

Bruguiera 223 

Bryozoa 188, 189 

Buckland, F. 12 

by-catch 163, 248, 250 

,4 C method see carbón-14 
CaC0 3 see calcium carbonate 
calanoid copepods see copepods 
Calanus 77 , 83, 95, 98-99, 102 , 120 
calcification 142, 216, 217, 219-220, 

calcite 142 

calcium carbonate 45, 78, 82, 141-143, 
213, 214, 216 
Calidris 155, 212 
California 241, 253, 257 
California Current 34, 62, 108-109 
Calyptogena 240-241 
Cambrian 157 

Cañada 130, 171, 197, 202, 204-205 

Canaries Current 34, 62 

capelin 158, 163 

carbohydrates 46, 47, 128, 134 

carbón cycle 141-143 

carbón dioxide 46-48, 55, 136, 139, 

140, 141-143, 260 

carbón-14 method 48-49 
carbonate ions 47, 141-143 
Cariaco Trench 226 
Caribbean Sea 63, 149, 219, 222, 226, 

carotene 47 
catadromous fish 162 
catch-per-unit-effort 249 
cellulose 43, 128, 134, 135, 211 
Celtic Sea 64- 65, 99 
Centropages 83 

Cephalopoda 148-149, 187 see also 
specific types 
Ceratium 39 , 43 
Cetácea 149-152 
Cetorhinus 157 

CFP see ciguatera fish poisoning 
Chaetoceros 39, 40, 41, 130 
Chaetognatha/chaetognaths 77 , 80 -81, 

Challenger expedition 7-10, 94, 227 
Chemical composition of seawater 26 
chemoautotrophs 136-737 
chemosynthesis 46, 136, 206, 233, 

Chile 164 , 147, 172 
China 164 , 174, 202 
China Sea 149 
Chironex 79 
chlorine 256 
Chlorophyceae 39 

chlorophyll(s) 47, 48, 49, 64- 65, 67, 
68-70, 179 

chlorophyll maxima 70 
cholera 174, 255 
Chondrichthyes 157-158, 162 
Chondrus 179 
Chordata 77, 183 , 189 
Chrysophyceae 39 , 45 see also 
Chthamalus 199-200 
ciguatera fish poisoning 44-45 
ciliates 77, 78-79, 182, 184 
circulation, oceanic see also currents 
bottom/deep 31, 32, 33, 226-227 
surface 34-35 
vertical 30-32, 35 
circumglobal species 101-102 
Cirripedia 183 , 190 see also bamacles 
Cladocera 77, 83, 84-85 
Cladorhizidae 231 

clams 44, 119, 172, 206-207, 216, 235, 
237, 240-241, 257 
Cliffs of Dover 45 
climate change 24, 103, 108-109, 

164-167, 205, 216, 260, 262-263 
see also regime shift 
Clio 11 

Clione 77, 82, 101, 103 

Cnidaria 77 , 79, 80, 183 , 185, 213, 216, 
228, 241 

C0 2 see carbón dioxide 
coccolithophorids 45 
cod 88, 89, 158, 163, 167, 170-77/, 249 
Coelenterata/coelenterates 77 see also 

coexistence, of phytoplankton 55-57 
coid core rings see rings 
coid seeps 241-242 
collection methods 

for benthos 191, 226, 231-232 
for phytoplankton 39 
for zooplankton 74-76 
colour patterns see also 

bioluminescence; counter-shading; 
of algae 179-180 
of fish 159 
of zoobenthos 230 
of zooplankton 91, 92, 93 
community, definition 5 
compensation depth 21, 53 
compensation light intensity 21, 50-51 
compensation point 50 
competition 55-57, 118-119, 122, 148, 
151, 152, 192, 198, 199-202, 204, 
217, 221, 260 

Computer simulations 13, 131-134, 171, 

Conchoecia 77 , 83 
con servad ve properties 26 
continental shelf 3, 63 
convergent continental fronts see fronts 
convergent gyres see gyres 
copepodite stages 83 
copepods 77 , 82-84, 97, 106, 241, 257 
see also Calanus', Metridia; 
calanoids 82-83, 109 
cyclopoids 83-84, 190 
harpacticoids 84, 190, 211, 228, 230 
copper 130, 254, 255 
coral(s) 185, 214-215, 216 see also 
coral reefs 
ahermatypic 217 
hermatypic 217 
coral mining 261 

coral reefs 60, 63, 213-222, 260-263 
see also coral(s) 
formation 219, 220 
growth 219-221 
human impacts on 260-263 
nutrition 217-218 
productivity 218-219 
types 219 
zonation 221-222 
coralline algae 178, 216, 222 
Coscinodiscus 39 , 40, 41 


counter-shading 91 

CPUE see catch-per-unit-effort 

crabs 11, 147, 159, 190, 224, 241 

Cretaceous period 41 

Crinoidea/crinoids 183, 187-188, 228 

critical depth 52, 53 

critical phase 168 

crocodiles 149 

Crocodylus 149 

crown-of-thorns starfish see Acantilaster 
Crustácea 76, 77, 82-85, 94, 147-148, 
183, 190, 216, 228, 241 
Cryptomonas 39 
Cryptophyceae 39 

Ctenophora/ctenophores 77, 80, 257 
cumaceans 229, 232 
currents 31-36 

biological importance of 35-36 
bottom/deep 31-32, 226-227 
surface 34-35 
cuttlefish 148 
Cyanea 77, 79 
cyanide 261 

Cyanobacteria 39, 46, 123, 138, 179, 
198, 206, 211 

Cyanophyceae see Cyanobacteria 
cyclonic gyres see gyres 
cyclopoid copepods see copepods 
Cyclothone 159, 160 
cypris larvae 87 
cysts 43, 196 

damming of rivers 247, 250 
Daphnia 84 

dark reactions see photosynthesis 
Darwin, C. 7, 11, 190, 219, 220 
DDE 253 
DDT 253 

decapod crustaceans 77, 85, 183, 190, 

decomposition 112, 135, 136 
deep scattering layers 95-96 
deep-sea benthic ecology 226-243 
Delaware Bay 155 
De les seria 179 

demersal species 94, 147, 159, 232 
denitrification 138 
density, of water 1, 30-32, 33, 209 
and pressure 33 

controlling deep water masses 31-32 
máximum density 31 
deposit feeding 186, 187, 207, 230, 235, 

and ecological zones 2-3 , 20-21, 
90-94, 177-178 
average of world ocean 1, 233 
máximum of world ocean 1 
depth of mixing 52 -53 see al so mixed 
surface layer 

Desmophyceae 42, 43 
detergents 255 
determínate growth 125 
detritus 117, 121-122, 128, 178, 

203-204, 206, 210-212, 224-226, 
234-236 see also phytodetritus 
development, type of 193, 209, 237, 243 
diapause 100 
diatomaceous ooze 41 
diatoms 11, 40-41, 48, 123, 136, 206, 
208, 210 
centric species 41 
pennate species 41, 179 
reproduction of 41 
Dictyocha 39 

diel vertical migration 33, 43, 94-97, 
106-107, 159 
nocturnal 95 
reverse 95 
twilight 95 

dinoflagellates 42-45, 55, 76, 77, 179, 
206, 208 see also zooxanthellae 
reproduction of 42 , 43 
symbiotic species 179 see also 
toxic species 43-45, 258 
Dinophyceae 42, 43 
Dinophysis 42 , 43 
dioxins 254 
disease 169, 174, 204 
disphotic zone 20, 21, 93 
dissolved organic matter 121-122, 186, 
204, 218, 240 

diurnal vertical migration see diel 
vertical migration 

divergent continental fronts see fronts 
divergent gyres see gyres 
diversity see species diversity 
Dohrn, A. 11 

dolphins 149, 151, 152, 250 
DOM see dissolved organic matter 
domoic acid 44 
doubling time 54 
downwelling 31-32, 63, 67, 104 
dredging, effect on marine life 247, 258, 
259, 260-261 

driftnet fishery 148, 156, 250 
dugongs 149, 153, 211 

East Australia Current 34 
eastern boundary currents 34 
Echinodermata/echinoderms 109, 183, 
187-188, 207, 216, 242 
Echinoidea/echinoids 183, 187, 188, 228 
echinopluteus larva 87 
Echiura/echiurids 183, 184, 186, 228, 

echolocation 13, 151 
echo-sounding see sonar 

ecological efttciency 114 
ecosystem, definition 5 
Ectocarpus 178 
Ectoprocta 183, 188 
eddies 59 see also gyres; rings 
eels 162, 172, 213 
egestion 127 

El Niño 24, 62, 166, 262 
elasmobranchs see Chondrichthyes 
Emiliania 39, 45 

enclosed experimental ecosystems 

endemicity 229, 242 
energy budgets 119-120, 126-129 
energy flow see food chains; food webs 
England 134, 200 
English Channel 89, 165 
Enhydra see sea otters 
Enteromorpha 197, 210, 255 
enteropneusts 183, 184, 186-187, 228, 
229, 241 see also Hemichordata 
epibenthic sled 231-232 
epibenthos 94, 181 
epifauna 181, 196, 230 
epiflora 181 

epipelagic species 92, 101, 102 
epipelagic zone 3, 92 
equatorial currents 34, 63 
equatorial upwelling 63 
erosión 226, 259, 260 
estuaries 27, 29, 65-66, 100, 209-213, 

Euglenophyceae 39 
Eukrohnia 94, 102-103 
Euphausia 77, 84, 95, 99-100, 102, 
117-119, 147-148 
superba see krill 

euphausiids 77, 83, 84, 92, 93, 95, 96, 
105, 147-148, 173 
euphotic zone, definition 20-21 
Europe 44, 162, 174, 202 
eurybathic species 34 
euryhaline species 29, 212, 222 
eurythermic species 25 
Eustigmatophyceae 39 
Eutreptiella 39 

eutrophic water 57, 123, 139-140 
eutrophication 255, 258, 260 
Evadne 77, 83 
evaporation 26, 27 

evaporation/precipitation balance 26 -28 
evolution 1, 149, 153, 155, 156, 157, 
213, 216, 263 
excretion 121, 127, 138 
expeditions, oceanographic 10 
extinction coefficient of light 19-20, 
132, 134 

exudation 49, 121, 204 
Exxon Valdez 156, 204, 252 


f-ratio 139-140, 218 
faecal pellets 235 
Favella 77 
feather stars 187-188 
femtoplankton 4 

filter-feeding see suspensión feeding 
fish (teleosts) 118, 119, 156-174, 207, 
215, 217, 228-229, 241, 249-250, 
253 see also Agnatha; 
Chondrichthyes; fisheries; specific 
types of fish 
anadromous 162, 250 
catadromous 162 
deep-water species 159-162 
demersal 159 
disease 45 
eggs 88-89 
farms see mar iculture 
fecundity 89, 162, 167 
fluctuations in abundance 164-169 
growth 167-169 
larvae 88-89, 118, 167-169 
migrations 159, 162, 769, 170-772 
mortality 44, 45, 89, 167-169 
production see fisheries, yield 
size 159, 167-169 

fisheries 162-171, 226, 247-251, 258, 

effects on ecosystems 247-251, 

management 12-13, 162-167 
yield 116-117, 139-140, 163, 
165-166, 248-250 

fisheries oceanography 12-13, 162-171 
flatworms 184 , 185-186 
food chains 112-117, 779, 219 
food webs 117-123 
Antarctic 117-118 
North Sea 119-120 

Foraminifera 77, 78, 183, 228, 229, 230 

foraminiferan ooze 78 

Forbes, E. 7 

Fraser River 171-772 

freezing point 

of freshwater 31, 32 
of seawater 22, 24, 31 
French Polynesia 261 
freshwater 29, 31, 32, 152, 157, 162, 

171, 209, 212, 213, 259 
fringing reefs 219, 226, 222 
fronts 59, 62-67, 154 

convergent continental 62-63 
divergent continental 62 
island mass effect 66 
Langmuir circulation 66-67, 103, 104, 

plañetary 63, 170 
river-plume 65-66, 209 
shelf-break 63-65, 68, 104, 170 

fucoxanthin 47, 180 
Fucus 178, 779, 197, 210 
Funk Island 155 

Galápagos islands 149, 238, 262-263 

Gastropoda 183 , 187 

gelatinous zooplankton 92, 93 

generation time 54, 113 

Georges Bank 151, 249 

Gigantocypris 84 

Gigartina 179 , 180 

Glaucas 90, 91, 102 

global warming 109, 143, 263 

Globigerina 77 

Gnathophausia 83 

Gonyaulax 39 , 42, 43 

Gran, H. H. 11 

gravity, effects on Ufe 1, 96, 129, 134, 

grazing rate 127 
great auk 155, 156 
Great Barrier Reef 63, 213, 215, 219, 

greenhouse effect 143 
Greenland 7, 31, 202 
gregariousness 199 

gross primary productivity see primary 

growth efficiency 128-129, 135, 169 
gross 129 
net 129 
Guam 262 

guano birds 165-166 

Gulf of Alaska 61 

Gulf of México 197, 241 

Gulf of Panama 263 

Gulf of St. Lawrence 32 

Gulf Stream 34, 62, 63, 107 

Gulf War 252, 259 

Gymnodinium 42 , 43, 44 

gymnosomes 77 , 82 see also Clione 

gyres 34, 59-62, 107 

anticyclonic (convergent) 60-61, 67, 

cyclonic (divergent) 61 

H 2 S see hydrogen sulphide 
hadal zone 5, 177-178, 226-238 see 
also trenches 
hadalpelagic zone 3 
haddock 89, 158 
Flaeckel, E. 10, 11 
hagfish 157 
hake 158 

half-saturation constants 
of light 51 

of nutrients 54, 55, 57, 58, 136 
halibut 758, 159 
Halimeda 178 
Halobates 90, 91 

halocline 27 
Hardy, A. 12 

harpacticoid copepods see copepods 
Harvey, H. W. 12 
Hastigerina 78 
Flawaii 66, 263 
heavy metáis see metáis 
Hemichordata 183 , 184, 186-187 see 
also enteropneusts 
Hemiptera 90 
Hensen, V. 11 

herring 89, 151, 158, 163, 165, 171 
heteropods 77, 81-82 
Heterosigma 39 

heterotrophic production/heterotrophy 
42, 74, 136, 137 
history, of oceanography 7-13 
Hjort, J. 10 
HNLC areas 55 
holoplankton 74, 76-86, 257 
Holothuroidea 183 , 187, 788, 227, 229 
homoiothermic species 25, 129, 135, 150 
Hooker, J. 11 
horseshoe crabs 155, 789 
Huxley, T. 10 
Hydra 214 
Hydrobia 210 
Hydrodamalis 153 
hydrogen sulphide 136, 238-243 
hydroids see Hydrozoa 
hydrostatic pressure 30, 33-34, 95, 96 
and depth 33-34 
measurement of 33 
hydrothermal vents 238-243 
Hydrozoa/hydroids 183 , 185 
hypersaline environments 27 

ice 31-32, 197, 201 
Ice Age 216 

ice algae see algae, epontic 
Iceland 31, 202 
ichthyoplankton 88 
iguanas 149 

illumination see solar radiation; visible 

indeterminate growth 125 
India 164 

Indian Ocean 63, 101, 148, 149, 153, 
163, 229, 249, 261 
Indonesia 164 , 226 
Indo-Pacific 149, 216, 223 
infauna 180-181, 196, 205, 230 
infrared radiation 18, 21-22, 91 
ingestión rate 127-128 
insects 90, 198, 224 

International Council for the Exploration 
of the Sea 12 

International Whaling Commission 151, 


interstitial fauna 207-209 
inteitidal zone 3, 27, 177, 196-202, 
205-211, 222-225 

introduction of species see translocation 
of species 

ionic regulation 28-29, 41 
Ireland 228 
iron 55, 136, 138 
island mass effect 66 
Isochrysis 39, 45 

isopods 183 , 189, 198, 206, 211, 224, 

isotherms 23 

Isthmus of Panama 102, 149 

jackfish 158 
Janthina 90, 91, 102 
Japan 44, 147, 148, 149, 151, 164 , 174, 

jellyfish see medusae 

Á"-selection 5-6, 57-58 
kelp 178, 202-205, 235, 258 
keystone species 201, 204 
Korea 148, 164 , 202 

krill 62, 84, 99, 105-106, 117-118, 119 , 
147 see also euphausiids 
Kurile Islands 149 

Kurile-Kamchatka Trench 227, 228, 233 
Kuroshio 34, 51, 63 

Labrador Current 35, 63 
lagoons 27, 219, 220 
Lake Baikal 152 
Lake Michigan 172 
Laminaria 203, 204-205 
lampreys 157 

Langmuir circulation 66-67, 103, 104, 

lantem-fish see myctophid fish 
Larvacea 85, 92 
lecithotrophic larvae 193, 237 
lichens 198 

life history pattems 5-6 
light see bioluminescence; 

photosynthetically active radiation; 
solar radiation; visible spectrum 
light limitation 2, 21, 212 see also solar 

light reactions see photosynthesis 
lignin 134, 135 

Limacina 77 , 82, 101, 102 , 103, 105 
limpets see snails 
lipids 47, 140 
Lithothamnion 178 

littoral zone 3 , 177, 196-202, 205-207, 
209-211,224-225 see also 
intertidal zone 
Litio riña 198, 210 
lizards 149 

lobsters 190 
Lucifer 77 
luciferase 93 
luciferin 93 

mackerel 88, 151, 163, 165 
macrobenthos see macrofauna 
Macrocystis 178, 202-203 
macrofauna 182, 206-207, 227-228, 
229, 232 

macrophytes see algae, benthic 
major constituents of seawater 26 
Maldive Islands 261 
mammals, marine 28, 34, 148, 149-153 
manatees 149, 153, 211 
mangáis see mangroves 
mangroves 178, 222-226, 259-260 
Mariana Trench 233 
mariculture 147-148, 172-174, 

256-257, 258, 259 see also ocean 

marine mammals see mammals, marine 
marine snow 85 
marshgrasses 178, 209-210 
Mclntosh, W. C. 12 
Mediterranean Sea 102, 136, 151 
medusae 77 , 79, 148, 257 
megalopa larvae 87, 88 
Meganyctiphanes 83, 102 
meiobenthos see meiofauna 
meiofauna 182, 207-209, 211, 212, 215, 
228, 230, 232, 241 
mercury 156, 254-255 
meroplankton 74, 79, 86-89, 90-91, 

109, 193, 214, 257 
mesopelagic species 92, 93, 101 
mesopelagic zone 3, 92 
mesotrophic water 57 
metáis 251, 254-255 
methane 136, 239 
Metridia 97, 102 , 106 
microbenthos see microfauna 
microbial loop 121-123, 203-204, 210, 

microfauna 182, 212, 215, 232 
Microgloma 237 
Micromonas 39 
microphytes see algae, benthic 
Microsetella 83 

migration 149, 151, 154-155 see also 
diel vertical migration; fish 
migration; vertical migration 
Minamata disease 254 
mineralization 136-143 
mixed surface layer 24 -25 
mixing see circulation, oceanic; 

mixotrophy/mixotrophic production 43, 
76, 218 

Mnemiopsis 102, 257 

Modiolus 210 

Mollusca/molluscs 77 , 81-82, 183 , 187, 
197-198, 215, 216, 227, 228, 229, 

monoculture 173 
moonlight 20 
Moseley, H. N. 7 
moults, of Crustácea 235 
mudflats 206, 211-212 
mud-skippers 225 
mullet 91 

Murray, J. 7, 9, 10, 11 
Mussel Watch 259 

mussels 44, 119, 172, 173, 174, 197, 
198, 199, 201-202, 241, 242, 259 
myctophid fish 88, 159, 160 
mysids 77, 83, 85, 94, 207, 230 
Mysticeti 150-151 
Mytilus see mussels 

Nanomia 77, 80 
nanoplankton 4 , 45, 77, 79, 85, 

121-122, 185 
Nassarius 210 
nauplius larvae 83, 87 
near real-time data 170-171 
nekton 4, 147-153, 156-162 
nematocysts 79, 80, 185, 214, 218 
Nematoda/nematodes 183 , 185, 224, 

228, 230 

Nemertea 183 , 184, 185 
Neocalanus 98, 102 , 106 see also 
Neomysis 11 

Nereocystis 202, 203, 204 
neritic zone 3 

net primary productivity see primary 
neuston 90-91, 102 
new nitrogen see nitrogen 
new production 139-140 
New Zealand 172,, 202 
nitrate 32, 46, 47, 48, 55, 56, 57, 58 , 66, 
136, 137-747 
nitrification 138 
nitrite 48, 55, 137-138 

cycle 137-140 
new nitrogen 138-140 
recycled nitrogen 138-140 
nitrogen fixation 46, 138 
Nitzschia 40, 41 
Noctiluca 16, 77 

North Atlantic Deep Water 31, 33 
North Equatorial Current 34, 35 
North Pacific Current 63 
North Sea 109, 119-120, 168, 169, 171, 
191, 192 , 249 
Norway 156, 164 


Nucella 200 

number of species see species diversity 
nursery areas (for fish) 168, 769, 211, 

nutridme 55, 70, 139 
nutrients see also ñames of specific 

and photosynthesis 53-60 
availability of 2, 55, 57-58, 180, 209 
recycling of 112, 121-123, 136-141, 

ocean ranching 173 
oceanic zone, definition 3 
Octopus 148 
Odonticeti 151 
Oikopleura 11 

oil, as metabolic by-product 41 
oil/oil spills 156, 200, 204, 251 , 252, 259 
Oithona 77 , 83 , 84, 102 
oligohaline species 212 
oligotrophic water 57, 58 , 122, 138, 
139-140, 213, 225, 233 
Oncaea 84, 102 
Ophiuroidea 183 , 187, 188 , 228 
Oscillatoria 39 , 46 
osmoregulation 28 
osmosis 28 

Osteichthyes 157, 158-162 
ostracods 77 , 83, 84, 183 , 190, 228 
otters see sea otters 
overfishing 118, 149, 151-152, 156, 

166, 248-251, 261-262 
oxygen 32, 44, 46, 47, 206, 226, 232, 
240, 242, 260 

oxygen minimum layer 92, 122 
oxyluciferin 93 
Oyashio 63 

oysters 44, 172, 224, 250, 256-257, 259 

Pacific Ocean 24, 32, 34, 55, 62, 63, 67, 
68, 69, 84, 98, 101, 102-103, 109, 
148, 149, 153, 159, 163, 170, 216, 
219, 222 
Pakistán 259 
Paleozoic 102 
Panama Canal 149 
PAR see photosynthetically active 

paralytic shellfish poisoning 44 
parasites/parasitism 84, 157, 162, 186 
Parathemisto 77 , 84, 101, 102 
parthenogenesis 85 
particulate organic matter 121 
patchiness 103-107, 232 
PCB see polychlorinated biphenyl 
peat deposits 210 
pelagic environment, definition 2 
penguins 118, 119 , 153, 154, 155, 253 
Periophthalmus 225 

permanent thermocline see thermoclines 
Persian Gulf 259 
Perú 155, 164 , 165-166 
Perú Current 62, 68 , 107 
Peruvian anchovy fishery 165-166 
pesticides 13, 156, 251, 253-254, 255, 
258, 259, 260 

Petroleum hydrocarbons 251, 252 see 
also oil/oil spills 
pH of seawater 142 
Phaeocystis 45 
Philippines 44, 164 , 260, 261 
phosphate/phosphorus 32, 46, 47, 48, 55, 
66, 136-137 

photic zone see euphotic zone 
photoautotrophs 137 
photocytes 93 

photoinhibition 50, 57, 70, 90, 91 
photophores 93, 159 
photosynthesis 46-58, 140-141 
dark reactions 47, 50-51 
light reactions 47, 50-52 
maximal 50-51 
nutrients and 53-58 
solar radiation and 50 -53, 58-59, 62, 

photosynthetic pigments 47, 179-180 
absorption spectrum of 47 
photosynthetic quotient 140-747 
photosynthetically active radiation 16, 

77, 18, 47, 50 
phycobilin 47 
phycocyanin 47, 180 
phycoerythrin 47, 180 
Physalia 77 , 79, 80, 90 
phytodetritus 69, 122, 234, 235, 236 
phytoplankton 4, 11, 39-46 
and bacteria 121-122 
and depth 69-70 
coexistence of 55-57 
collection of 39 
experimental studies 130 
growth rates 50-58 
types of 39-46 
Piccard, J. 228 

picoplankton 4, 45, 51-52, 146 
pilchards 88, 89, 158, 163 , 165 
Pinguinus 155 
Pinnipedia 152 
Pisaster 201 -202 
plaice 89, 168, 169 
planetary frontal systems see fronts 
plankton see also phytoplankton; 
definition of 3 -4, 11 
size of 4 

plankton nets 74-75 
planktotrophic larvae 193 
plastics 251 , 252-253 

Platyhelminthes 183 , 185-186 
Pleistocene 103 
Pleurobrachia 77 , 80 
pleuston 90-91 
Pliocene 103 
Podon 77 , 83 

Pogonophora 183 , 184, 186, 228 
poikilothermic species 25, 129, 135 
pollock 158, 163 

pollutants/pollution 57, 65, 102, 156, 
174, 247, 251-256, 258-259, 260, 
262 see also specific types 
Polychaeta/polychaetes 77 , 81, 183 , 184, 
186, 207, 212, 216, 224, 228, 229, 
241, 242, 255 

polychlorinated biphenyl 156, 254, 259 
polyculture 173 
Polyplacophora 183 , 187 
POM see particulate organic matter 
population, definition of 5 
Porichthys 93 
Porifera 183 , 184-185 
Porphyra 180, 256 
porpoises 149, 151-152, 159 
Portuguese man-of-war see Physalia 
Postelsia 202, 203 
Prasinophyceae 39 
Precambrian 184 
predation 96-97, 106, 118-119, 
121-122, 151, 192-193, 200, 
201-202, 204, 207, 221, 231, 262 
predation rate 127 
pressure see hydrostatic pressure 
primary consumers 112 
primary producers 46, 112, 134 
primary production/productivity 46-70, 
annual cycles 58, 59, 68-69 
benthic 180, 201, 203-204, 206, 
210-211, 218-219, 225 
definition 46, 48 

effects of grazing on 57, 68-69, 200, 
201, 204 

global valúes, pelagic 67-70, 116 , 
gross 50 

measurements of 48-49, 180 
net 50 

physical Controls of pelagic 58-67 
terrestrial 67-68, 134-136 
Prochlorococcus 46 
prochlorophytes 46 
production see primary production; 

secondary production 
production to biomass ratio 135 
Prorocentrum 42, 43 
protein 47, 78, 135 
protists 76 

Protoperidinium 39 , 42, 43 


Protozoa 76-79, 121-122, 182, 183-184 
see also specific types 
Prymnesiophyceae 39, 45 
Prymnesium 39, 45 
Pseudocalanus 83, 102 
Pseudonitzschia 44 
pseudothecosomes 82 
PSP see paralytic shellfish poisoning 
pteropod ooze 82 
pteropods 82 see also thecosomes 
pycnocline 24, 92, 95, 107, 122 
Pyrodinium 44 
Pyrosoma 77 
Pyrrophyceae 39, 42 

r-selection 5-6, 57, 86, 243 
Raben, E. 12 

radiation see infrared radiation; solar 
radiation; ultraviolet radiation 
radioactive wastes 251, 255-256 
Radiolaria 9-10, 77, 78, 79, 136 
radiolarian ooze 78 
Raphidophyceae 39 
rays 28, 157-158 
recolonization experiments 237 
recycling of nutrients see nutrients 
red knots 155, 212 
Red Sea 24 , 27, 102 
red tide 43-44, 258 
reef crest 221-222 
reef flat 221 

refractory material 136, 211 
regenerated production 138-140 
regime shift 108-109, 164-166 
remóte sensing 13, 49, 67, 167, 170 
remotely operated vehicles 76 
reptiles 149 

respiration 46-47, 113, 127, 128, 142, 
219, 237 
resting spores 41 
Rhincodon 157 
Rhizophora 223 
Rhizosolenia 39 
Rhodella 39 
Rhodophyceae 39 
Riftia 239-240, 241, 243 
rings 59, 60-62, 104, 107 
coid core 61, 62 
warm core 61, 62 
Ritter, W. 11 

river-plume fronts see fronts 
Rockall Trough 228 
rocky intertidal shores 198-202 
zonation on 198-200 
Ross, James 7 
Ross, John 7 

ROV see remotely operated vehicles 
Russell cycle 164-165, 166 
Russia 153, 164 

Sagitta 77, 81, 102, 105, 165 
salinity 25-29, 197 
and depth 27, 29 
biological importance of 28-29 
measurement of 25 
surface, distribution of 26-27 
salmón 89, 148, 162, 170, 171, 172, 173, 
211, 213, 250 
Salpa 77, 102 
salps 77, 85-86, 148 
saltmarshes 178, 109-210 
sampling see collection methods 
sand bars 211 
sand beaches 205-209 
sand dollars 187, 207 
sardines 89, 158, 163 
Sargasso Sea 39, 61, 67, 68, 91, 107, 
162, 236 

Sargassum 39, 67, 91, 134 
satellite imagery see remóte sensing 
saxitonin 44 

Scaphopoda 183, 187, 189 
Scilly Isles 66 
Scotland 200 
scuba diving 13, 76 
sea anemones 183, 184 , 185, 214, 227, 
228, 230, 241 
sea cows 149, 153, 203 
sea cucumbers 187, 188, 207, 224, 227, 
230, 260 
Sea Empress 252 
sea level changes 216 
sea lilies 183, 187-188, 228 
sea lions 149, 152, 253 
sea otters 204 
sea snakes 149 
sea turtles 149, 253 

sea urchins 183, 187, 188 , 204-205, 207 
sea wasp 79 

seabirds 28, 44, 91, 118, 148, 153-156, 
165-166, 212, 250, 253 see also 
specific types 

seagrasses 134, 178, 210-211, 216, 221, 

seáis 118, 119, 149, 152 
seamounts 219 

seasonal thermocline see thermoclines 
seastars see starfish 
seaward slope, of coral reefs 222 
seawater composition see Chemical 
composition of seawater 
seaweeds 172, 178, 179-180, 210 see 
also algae, benthic 
secondary consumers 112 
secondary production 112 
benthic 191-192, 232-233 
measurement of 113-117, 123-134, 

pelagic 113-117, 123-134, 135-136 

self-shading 68 

Sergestes 77 

sessile, definition of 184 

sewage 65, 174, 251, 255, 258, 259, 

260, 262 

sharks 28, 148, 157-158, 254 
shelf-break fronts see fronts 
shrimp 85, 94, 147, 172, 190, 228, 229, 

shrimp fisheries 250 
sigma-t 31 

silica (Silicon) 40-41, 45, 48, 55, 78, 136 
silicoflagellates 45, 136 
Singapore 172 
sinking, retardation of 41 
sinking rates 234-235 
of faecal pellets 235 
of phytoplankton 41, 234 
of zooplankton 234 
siphonophores 77, 79, 80, 92, 93 
Sipuncula/sipunculids 183, 184, 186, 

227, 228, 230 
Sirenia 153 
size 113 

of nekton 4, 148, 150, 159, 162, 167 
of plankton 4, 74, 75, 92, 134-135 
of zoobenthos 182, 228, 230, 239-240 
skates 157-158 
Skeletonema 40, 4 1 

snails 187, 198, 200, 201, 202, 207, 210, 
211, 216, 224, 229, 241, 254 ^ 
also specific types 
snakes see sea snakes 
solar radiation 16-21, 58-59 
and depth 2, 18-21 
and latitude 17, 18, 19, 58-59 
and photosynthesis 50-53, 58-59 
and season 17, 18, 19, 58-59 
at the sea surface 76-18, 90, 91 
diel/diurnal variation 18, 19 
effect of clouds on 18 
energy-wavelength spectrum 17, 

18-20, 179-180 
meásurements of 17-18 
solé 159, 172 
sonar 13, 76, 147 
South Africa 202, 203 
South Equatorial Current 34 
South Sandwich Trench 233 
Southern Ocean see Antarctic Ocean 
Spain 174 

species, definition of 4-5 
species composition 57, 123, 192, 201, 

species diversity 5, 177, 192, 201-202, 
206, 210, 212-213, 255, 260 
marine vs. nonmarine 1 


of benthos 177, 183, 184, 185, 186, 
187, 188, 190, 191, 215, 
231-232, 239, 242 
of coráis 215-216 
of fish 157, 159, 161, 177, 215 
of mammals 150, 151, 152, 153, 177 
of phytoplankton 40, 43, 57 
of seabirds 153, 215 
of zooplankton 76, 78, 81, 82, 84, 85, 
101, 103 

species succession 51, 55-57 
splash zone see supratidal zone 
sponges 183, 184-185, 216, 227, 228, 

squid 118, 119, 148-149, 758, 159, 170 
St. Lawrence Estuary 152 
standing stock 48, 113, 180 
starfish 183, 187, 201-202, 207, 227, 


Steller, G. W. 153 
stenobathic species 34 
stenohaline species 29, 212-213 
stenothermic species 25 
stock/recruitment theory 164, 166 
stomiatoid fish 159, 160 , 161 
Strait of Juan de Fuca 171-772 
stratification, of water 24-25, 43, 59, 
63-64, 70, 109 
stratification Índex 63-65 
stromatolites 179 
Strombidium 11 
Stronglyocentrotus 204-205 
Stygiopontius 241 
sublittoral zone 3, 177, 202 
submarines/submersibles 13, 76, 226, 


subtidal zone 177, 202-205, 207, 
210-211, 225 see also sublittoral 

succession see species succession 
Suez Canal 102 
sunlight see solar radiation 
supralittoral zone 3, 177, 198, 206 
supratidal zone 177 see also supralittoral 

suspensión feeding 184-185, 187, 188, 
189, 190, 207, 218, 228, 230-231, 

swimming speeds 
of squid 148 
of zooplankton 95 
Symbiodinium 217 
symbiosis 217-218, 239, 240-241 
Synechococcus 39, 46 

Tahiti 197 
Taiwan 148 

tanaids 183, 189, 190, 228, 229, 230 
TBT see tributyltin 

teleosts 28, 157 see also fish (teleosts) 

temperature 21-25, 30-32, 108-109, 
197, 202, 206, 213, 226, 238, 241, 
242, 256 

and biogeographic zones 22-23 
and latitude 22-24 
and season 21-25 
at the sea surface 21-24 
distribution of with depth 24-25 
of air 1, 22, 24 
of máximum density 31 
temperature-salinity diagrams 37 
temperature tolerance 25 
tertiary consumers 112 
Tethys Ocean 102 
Tetrasalmis 39 
Thailand 164, 261 
Thalassia 210, 225 
Thalassiosira 40, 41 
thecosomes 77, 82 see also Limacina 
Themisto 83 

thermal effluents 251, 256 
thermoclines 24-25, 60, 61, 63, 66, 95, 
107, 109 

permanent 24-25, 59, 92 
seasonal 24-25 
Thiomicrospira 239 
Thompson, J. V. 11 
Thomson, C. W. 7, 8 
Thysanoessa 83, 102 
tides 197, 209, 213, 221-222, 224 
Tindaria 237, 240 
tintinnids 78-79 
Todarodes 149 
Tomopteris 77, 81, 102 
toothed whales 150, 151-152 
Torrey Canyon 200, 252 
transfer efficiency 114, 126 
translocation/transfers of species 172, 

transparency 80, 82, 91, 92 
trenches 177-178, 227-230, 233 
tributyltin 254, 259 
Tridacna 216, 218 
tritons 262 

trochophore larvae 87, 88 

trophic levels 112-117 

tuna 89, 148, 158, 159, 170, 250, 254 

tunicates 183, 189-190, 231 

turbidity 213, 260 

turbot 159, 172 

turbulence 24-25, 41, 64, 107, 131, 180, 

turtles 28, 148, 149, 211 
Vea 210 

ultraviolet radiation 18, 91 
Viva 179, 210, 255 

upwelling 32, 60, 61, 62, 63, 66-67, 68, 
100, 104, 107, 115-116, 139, 154, 
202, 226, 232 

Antarctic 62 

coastal/continental 62, 68, 154, 
equatorial 63 

off western Africa 62, 100 
off western South America 62, 

urea 127, 137-139 

Vampyroteuthis 92 
Velella 90, 91 
veliger larvae 87 
vertical distribution 
of fish 159-162 

of zooplankton 90-100, 106-107 
vertical environmental gradients 2 
vertical migration 70, 235 see also diel 
vertical migration 
biological significance of 70, 235 
seasonal 98-100 

Vestimentifera 183, 186, 239-241 see 
also Riftia 
viruses 4, 123, 255 
visible spectrum of light 16, 17, 18 
visión 17, 18, 20, 82 
vitamins 48, 55, 148 
viviparous plants 223 
volume of ocean 1 

walruses 149, 152 
Walsh, D. 228 
warm core rings see rings 
water masses 30-33 
western boundary currents 34 
whales 118-119, 149-152, 254 
whaling 118, 147, 151-152 

Xanthophyceae 39 
xanthophyll 47 

Xenophyophoria 183, 184, 228 

zebra mussel 257-258 
zoea larvae 87, 88 
zonation, benthic 
of algae 179-180, 200 
on rocky shores 198-200 
on sand beaches 206-207 
zoobenthos 180-191 
zooflagellates 76, 122 
zooplankton 4, 74-109 
collection methods 74-76 
energy budgets of 126-129 
fecundity of 129 
growth of 124-125, 127-129 
size distribution 4, 74 
types of 76-89 
vertical distribution 90-100 
zoogeography 100-103 
zooxanthellae 214, 217-218, 220, 260 
Zostera 210-211 

■ :< 

.. :í_ - 

Platas Hnotlageilates:the 
lona) 1* 8 species ol the 

Píate 4 MAIN PHOWGRAPH 4 PBd tlde olí the cahsl 

of Japan causea by a bloom of dlnoflagellates. 

*<**■ ' 

Píate 5 4 coccolithophorM, Scyptiespluera apstakil, 
with calcapeous platas known as coccolitln. 
Dlametep, < 20 pm. 

::. -<**--v* ■* -r*- 1 m 1, ¿ - | »»■ 

Píate 8 4 photosynthetic sillcoflagellate, Dlctyoclu 
fíbula, with a skeleton formed of sílice. Width, 
inctuding splnes, ea. SO pm. 

nata 9 4 padiolarian, 
Astromma arislotetis, 
with a skeleton formed 
el siliceous spicules. 
Dlameter Includlng 

Plata 11 

tas «xtortfal test and tí* 
hrown lood spot» wfttiln 
llw profanan, and ttie 
ctttprofecting on tlu 
rlgtit stta of ttie phato. 

I ength, ca. 200 pm. 

raghm (8). Dlftarwt 

dtner ent surface 
temperatur es: red, 
warmoat; pü, cotdest. 

Píate 12 The medusa 
Pelagia noctiluca. Note the 
slender* tralllng tentacles and 
largor central, yellow oral 
lobes. Bell dlameter about 


Píate 13 A colonial 
siphonophore ( Agaima). Note 

Píate 17 A heteropod 

[Carinaría] with 

swlmtning fin. 
«Mi is dlrected 

Pialéis Afllamjosomatous 
pieropod (CavierUlH^i^ a clgar- 
shaperi Shell (about 10 mm fono) ‘ 
and paired swimming wings. 

Píate 20 A pseudothecosome 
mollusc ( Coroila ) with an expanded 
wingplate lying between the lower 
rounded body and uppei* proboscis. 
Wingspan about 5 cm. 

Píate 21 The gymnosome Cltone limacina, a mollinean 
predator ol species of limacina (thecosomes). Ngte the 
shell-less streamlined body and paired swimming wings. 
Length 3-7 cm. # . 

« t 

Píate 22 The siphonophore Forskaiia with attached 
amphipods that appear as white round spots on the upper 
swimming bells. Múltiple tentacles project below. ' 

Píate 23 A deep-sea shrimp IPasiphaea) showing the red 
coloratioii that is typical ol many mesopelagic zooplanhton. 
Scaleinmm. * •* . • 

*♦ ( * a 

Píate 24 An appendlcularian (larvacean), 

Stegasoma magnum, which has been dyed jwith carmine . 
partieles te show the (red) filters ot its transparent 
mucous house. The animal ¡tselMs Jocated centrally, with 
a small white body and slender t?’il.*Size about 3 cm. 

• f * ‘ * 

• * i * • 

Píate 25 The solitary stage of the salp Pegas soda 
(lengtb, ca. 25 mm). It will bud aseitually to* produce %a 
Chain ol individuáis shown in Píate 26. 


• 4 | 

*• « * C. v 

Píate 29 The neustonic nudibranch moliese 
Glaucas atlánticas. Its fingerlike cerata contain 
stinging nematocysts whlch the animal obtains 
by leeding on Vetolia and Phvsatia. To 30 mm 

Píate 23 The aggregate 
stage el the salp Pegea seda 
in which the individuáis ape 
gpouped in chaina. Chain 
lengtlLca. 13 cm. 

27 A common 

biológica) lineages 

Píate 33 An aerial 
view of the Fraser 
River estuary in Brit 
Columbia, Camila. 
Several branches al 
Fraser River run 

discharge large 
voluntes ol sílt-laden 
treshwater into the 
Pacific Ocean. Ihe. . 
locations of aeveral of 
the estirarme 
communities can be 
4een including the 
ifÁrrow saltmarsh ron 
(8M), a seagrass bed 
(80, the eNtensivB 
fMidfots (MF), and the 
ftfbnlc community (P). 

Calyptagena, a large clam (to 4 9f 
cm) that forma a majar portipn ,* 

polyps contato bañarías of 
nematocysts osad in leed 

types of cnidarians (a 
hydruiaiú tound in 
coráis on tropical raéis. 

Píate 41 An aerial vlew el the oil spilled 
Irom the tanker Amoco Cádiz, Corning 
ashore at Roseen, France, in 1978. Note 
that although olí covers most ol the 
loreshore, beach areas lo the lee ol the 
islands and bays are relativaly free ol oil. 

Píate 38 Tropical mongrove trees 
witti extensive root systems. 

Píate 39 Rittia pachyptila, a vestimentileran 
tubeworm that is a dominant animal at the Galápagos 
hyrfrottiermal vents. A plome ol red tentacles projeets 
Irom the leathery tobe ol each individual.